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'.
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