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 #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
Merge pull request #1597 from take-cheeze/mrbcfile

debug record size assertion fail
Fix build error for MRB_WORD_BOXING
Merge pull request #1599 from kyab/fix_mrb_word_boxing

Fix build error for MRB_WORD_BOXING
float objects in pool are objects when MRB_WORD_BOXING is set
rename mrb_intern2() to mrb_intern(); huge API incompatibility; close #1513