Clang compiler support

[topilski]

Hi, please add clang compiler support, bcuz on MacOSX in mostly used clang as compiler. In code you can replace #if defined(COMPILER_GCC) to #if defined(COMPILER_GCC) || defined(COMPILER_CLANG) and this should work, also need to detect is compiler clang.

[anrieff]

Your fork already has it, right? In lieu of issue #96, I might just integrate that.

[orivej]

This might have been fixed by #112. (For me even libcpuid 0.4.0 builds on Linux and macOS with Clang, but I'm using stock Clang that defines __GNUC__, not Apple Clang.)

[anrieff]

Yes, I can confirm that libcpuid now compiles fine with clang 3.7.0 on Linux. Please report further issues.

[anrieff]

And, of course, thanks for your contribution, @orivej :)