Better support for installations with split dir setups

[DerDakon]

Please use CMAKE_INSTALL_FULL_LIBDIR and friends in pkg-config files like here:

Imath/config/CMakeLists.txt

Line 42 in ae4e3af

set(libdir "\${exec_prefix}/${CMAKE_INSTALL_LIBDIR}")

This will allow a user to specify e.g. an install directory outside $prefix and the .pc file will still work.

I just noticed this in a downstream Gentoo patch.

[cary-ilm]

Check out #172, I think this is right. It passes my testing, but I'm not sure about the case that tripped you up.

For OpenEXR v2.5 (which predates the separate Imath library/repo), I also submitted this: AcademySoftwareFoundation/openexr#1061

[cary-ilm]

Closing this now, assuming it's addressed by #172. Let us know if there are outstanding issues.