opencl-clang: explicitly set GITHUB_PROJECT

This works around a subtle bug in github.mk.

Long story short is GITHUB_PROJECT is used to infer WRKSRC when github.mk
is in use. In addition, GITHUB_PROJECT is inferred from DISTNAME when
not explicitly set by the package.
If one wants to reference WRKSRC in a BUILDLINK_TRANSFORM, that breaks
because cwrappers.mk is evaluated before bsd.fetch.mk and GITHUB_PROJECT
is not yet known at that point, leading to an invalid WRKSRC expansion.

(tnn)