add ruby and C code execution order note

Always include build_config.rb to compile dependency.

add MRB_ prefix to STRINGIZE macro

Add target example for Intel Galileo board

limit preserved significand length

add 'd' format specifier in mrb_get_args and mruby-random gem improvement

Issue minor corrections and updates to INSTALL file.

Add error for C99 style variable declaration on travis-ci

build_config.rb modifies `cc.defines` (eg. `DISABLE_GEMS`) so files depending on such flags should be rebuilt.

Ref. (Failing scenario): https://gist.github.com/cho45/9181191

Add warning for C89-style variable declarations

Make mruby build directory customizable.

Fix unused function warning when readline enabled (mirb)

preserve significands in float-string conversion

Fix possible arena overflow in mruby-hast-ext.

add tests for String#capitalize!, String#downcase!, and String#upcase!