Add documentation for compilation process

doc/compile/README.md

+# Compile
+
+mruby is using Rake to compile and cross-compile all libraries and
+binaries.
+
+## Prerequisites
+
+To compile mruby out of the source code you need the following tools:
+* C Compiler (i.e. ```gcc```)
+* Linker (i.e. ```gcc```)
+* Archive utility (i.e. ```ar```)
+* Parser generator (i.e. ```bison```)
+* Ruby 1.8 or 1.9
+
+Optional:
+* GIT (to update mruby source and integrate mrbgems easier)
+* C++ compiler (to use GEMs which include *.cpp)
+* Assembler (to use GEMs which include *.asm)
+
+## Usage
+
+Inside of the root directory of the mruby source exist a file
+called *build_config.rb*. This file contains the build configuration
+of mruby and looks like this for example:
+
+```
+MRuby::Build.new do |conf|
+  conf.cc = ENV['CC'] || 'gcc'
+  conf.ld = ENV['LD'] || 'gcc'
+  conf.ar = ENV['AR'] || 'ar'
+
+  conf.cflags << (ENV['CFLAGS'] || %w(-g -O3 -Wall -Werror-implicit-function-declaration))
+  conf.ldflags << (ENV['LDFLAGS'] || %w(-lm))
+end
+```
+
+All tools necessary to compile mruby can be set or modified here.
+The following options can be configurated:
+
+* conf.cc (C compiler)
+* conf.ld (Linker)
+* conf.ar (Archive utility)
+* conf.cxx (C++ compiler)
+* conf.objcc (Object compiler)
+* conf.asm (Assembler)
+* conf.yacc (Parser Generator)
+* conf.gperf (Hash function Generator)
+* conf.cat (Concatenate utility)
+* conf.git (GIT content tracker)
+* conf.cflags (C compiler flags)
+* conf.ldflags (Linker flags)
+* conf.cxxflags (C++ compiler flags) 
+* conf.objccflags (Object compiler flags)
+* conf.asmflags (Assembler flags)
+* conf.gem (A GEM which should be integrated - can be set several times)
+
+To compile just call ```./minirake``` inside of the mruby source root. To
+generate the test tool environment call ```./minirake test```. To clean
+all build files call ```./minirake clean```.
+
+### Cross-Compilation
+
+mruby can also be cross-compiled from one platform to another. To
+achive this the *build_config.rb* needs to contain an instance of
+```MRuby::CrossBuild```. This instance defines the compilation
+tools and flags for the target platform. An example could look
+like this for example:
+
+```
+MRuby::CrossBuild.new('i386') do |conf|
+  conf.cc = ENV['CC'] || 'gcc'
+  conf.ld = ENV['LD'] || 'gcc'
+  conf.ar = ENV['AR'] || 'ar'
+
+  if ENV['OS'] == 'Windows_NT' # MinGW
+    conf.cflags = %w(-g -O3 -Wall -Werror-implicit-function-declaration -Di386_MARK)
+    conf.ldflags = %w(-s -static)
+  else
+    conf.cflags << %w(-g -O3 -Wall -Werror-implicit-function-declaration -arch i386)
+    conf.ldflags << %w(-arch i386)
+  end
+end
+```
+
+You can configurate the same options as for a normal build.
+
+## Build process
+
+During the build process the directory *build* will be created. The
+directory structure will look like this:
+
+```
++- build
+   |
+   +-  host
+       |
+       +- bin          <- Binaries (mirb, mrbc and mruby)
+       |
+       +- lib          <- Libraries (libmruby.a and libmruby_core.a)
+       |
+       +- mrblib
+       |
+       +- src
+       |
+       +- test         <- mrbtest tool
+       |
+       +- tools
+          |
+          +- mirb
+          |
+          +- mrbc
+          | 
+          +- mruby
+```
+
+The compilation workflow will look like this:
+* compile all files under *src* (object files will be stored 
+in *build/host/src*
+* generate parser grammar out of *src/parse.y* (generated
+result will be stored in *build/host/src/y.tab.c*
+* compile  *build/host/src/y.tab.c* to  *build/host/src/y.tab.o*
+* create *build/host/lib/libmruby_core.a* out of all object files (C only)
+* create ```build/host/bin/mrbc``` by compile *tools/mrbc/mrbc.c* and
+link with *build/host/lib/libmruby_core.a* 
+* create *build/host/mrblib/mrblib.c* by compiling all *.rb files
+under *mrblib* with ```build/host/bin/mrbc```
+* compile *build/host/mrblib/mrblib.c* to *build/host/mrblib/mrblib.o* 
+* create *build/host/lib/libmruby.a* out of all object files (C and Ruby)
+* create ```build/host/bin/mruby``` by compile *tools/mruby/mruby.c* and
+link with *build/host/lib/libmruby.a*
+* create ```build/host/bin/mirb``` by compile *tools/mirb/mirb.c* and
+link with *build/host/lib/libmruby.a*
+
+### Cross-Compilation
+
+In case of a cross-compilation to *i386* the *build* directory structure looks
+like this:
+
+```
++- build
+   |
+   +-  host
+   |   |
+   |   +- bin           <- Native Binaries
+   |   |
+   |   +- lib           <- Native Libraries
+   |   |
+   |   +- mrblib
+   |   |
+   |   +- src
+   |   |
+   |   +- test          <- Native mrbtest tool
+   |   |
+   |   +- tools
+   |      |
+   |      +- mirb
+   |      |
+   |      +- mrbc
+   |      | 
+   |      +- mruby 
+   +- i386
+      |
+      +- bin            <- Cross-compiled Binaries
+      |
+      +- lib            <- Cross-compiled Libraries
+      |
+      +- mrblib
+      |
+      +- src
+      |
+      +- test           <- Cross-compiled mrbtest tool
+      |
+      +- tools
+         |
+         +- mirb
+         |
+         +- mrbc
+         | 
+         +- mruby
+```
+
+An extra directory is created for the target platform. In case you
+compile for *i386* a directory called *i386* is created under the
+build direcotry.
+
+The cross compilation workflow starts in the same way as the normal
+compilation by compiling all *native* libraries and binaries.
+Aftwards the cross compilation process proceeds like this:
+* cross-compile all files under *src* (object files will be stored 
+in *build/i386/src*
+* generate parser grammar out of *src/parse.y* (generated
+result will be stored in *build/i386/src/y.tab.c*
+* cross-compile  *build/i386/src/y.tab.c* to  *build/i386/src/y.tab.o*
+* create *build/i386/mrblib/mrblib.c* by compiling all *.rb files
+under *mrblib* with the native ```build/host/bin/mrbc```
+* cross-compile *build/host/mrblib/mrblib.c* to *build/host/mrblib/mrblib.o*
+* create *build/i386/lib/libmruby.a* out of all object files (C and Ruby)
+* create ```build/i386/bin/mruby``` by cross-compile *tools/mruby/mruby.c* and
+link with *build/i386/lib/libmruby.a*
+* create ```build/i386/bin/mirb``` by cross-compile *tools/mirb/mirb.c* and
+link with *build/i386/lib/libmruby.a*
+* create *build/i386/lib/libmruby_core.a* out of all object files (C only)
+* create ```build/i386/bin/mrbc``` by cross-compile *tools/mrbc/mrbc.c* and
+link with *build/i386/lib/libmruby_core.a* 
+
+## Test Environment
+
+mruby's build process includes a test environment. In case you start the testing
+of mruby, a native binary called ```mrbtest``` will be generated and executed.
+This binary contains all test cases which are defined under *test/t*. In case
+of a cross-compilation an additional cross-compiled *mrbtest* binary is 
+generated. This binary you can copy and run on your target system.