No pkg-config support on Linux

[gfxstrand]

Embree currently provides support for find_package() with cmake but that's it when it comes to package discovery. This makes it painful to use as a dependency if the project which depends on it uses any other build system. On Linux, the pkg-config system is fairly universal. It would be really nice if embree would install an embree.pc file.

[cmarcelo]

Related PR about this #253.