Use 'gluegen-clang.properties' for generic clang and 'gluegen-xcode_clang.properties' for OSX xcode-clang ; Add GNU/Linux LLVM/clang build scripts

Use 'gluegen-clang.properties' for generic clang and 'gluegen-xcode_clang.properties' for OSX xcode-clang.

Add GNU/Linux LLVM/clang build scripts
glibc-compat-symbols.h: Use GLIBC_2.0 for 32bit ; Use asm(..) instead of __asm__(..) to support clang ; Branch on OS predef-macro for GLIBC detection, allow __GNUC__ and __clang__ on __linux__