declare struct mrb_irep.
Merge pull request #1590 from iij/pr-declare-struct-mrb-irep

declare struct mrb_irep.
Make mrb->arena variable sized.  Use MRB_GC_FIXED_ARENA for old behavior.
You will not see "arena overflow" error anymore, but I encourage gem authors
to check your gems with MRB_GC_FIXED_ARENA to avoid memory broat.
build with MRB_GC_FIXED_ARENA.
Automatically detect toolchain if it is visualcpp, and hence we drop vs2010.rake altogether - it’s no longer needed.
Merge pull request #1591 from iij/pr-fixed-arena-on-travis

build with MRB_GC_FIXED_ARENA on travis
Merge pull request #1592 from perlun/auto-toolchain

Automatically detect toolchain if it is visualcpp, and hence we drop vs2010.rake altogether - it’s no longer needed.
Updated the documentation to better match the way things work now that 172cd28 is merged.
Modify build script for gcc.

Newly added compile option '-std=gnu99' is not necessary for C++.
And 'g++' should be used to compile for C++ source codes, I think.
Modify referenced environment variables.

'CXX' is used instead of 'CC'.
'CXXFLAGS' is used instead of 'CFLAGS'.
add enable_debug method to MRuby::Build and build debug version in travis-ci
fix realloc error
Merge pull request #1593 from perlun/patch-1

Updated the documentation to better match the way things work now that 172cd28 is merged.
Merge pull request #1595 from take-cheeze/filename_realloc_fix

get_filename_table_size realloc fix.
remove MRB_PARSER_BUF_SIZE configuration; close #1596
Merge branch 'modify_toolchains_gcc' of https://github.com/crimsonwoods/mruby into crimsonwoods-modify_toolchains_gcc
add CFLAGS as a fallback from CXXFLAGS; ref #1594
append defines instead of assigning
use mrbc of current build in non cross build
move debug record size assertion to correct position