remove direct references to architecture and platform in build extension

these should use the environment variables ${xc_platform}
${xc_architecture} from build_all.sh
don't rely on the systems c++ compiler use the one defined by $CXX

this is because the vesion of gcc provided by osx is actually clang
which leeds to some compatability problems (use a real gcc!!)

CXX is defined in build_all.sh
add comments to assist users in setting up a build of xcamshift