[PATCH libclc] Make libclc more Linux FHS conform.

- First introducing a versioning scheme
- Add --libexecdir, --includedir and --pkgconfigdir and prefill them as well as --prefix
- Build all targets by default
- Create clc.pc and install it in $pkgconfigdir
- Use clang++ instead of c++
- Rename builtins.bc to built_libs/$triple.bc and install them in $libexecdir
- Includes are installed recursively to $includedir
- Finally add $(DESTDIR) for 'make install'