Implement default Random instance.

small-optimization for mrb_time_zone

mrb_str_new_static is more efficient than mrb_str_new in this case.

Using mrb_str_new instead of mrb_str_new_cstr

Previously, the default random number generator was implemented using static
storage.  This storage is common to all instances of mruby in a process, and
use of the default random number generator in one instance will perturb the
default random number generator in other instances.  It is also not thread
safe.

With this change, the default random number generator is defined as
Random::DEFAULT, as it is in CRuby.

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!