add an alloy.py option to build ispc binary with g++
'--ispc_compiler' changed to '--ispc_build_compiler', 'check_isa.cpp' now compiles with '--ispc_build_compiler' (g++ or clang), removed code duplication
Merge branch 'master' of https://github.com/ncos/ispc into ispc_gcc_compile_option

Conflicts:
	alloy.py
changed 'ispc-compiler' to 'ispc-build-compiler' in error message
Merge pull request #821 from ncos/ispc_gcc_compile_option

Add an alloy.py option to build ispc binary with g++