Improve pkg-config file generation

[kmilos]

Just like w/ Imath, the .pc file generation should ideally deal with both absolute and relative paths in CMAKE_INSTALL_{INCLUDEDIR,LIBDIR} here:

openexr/cmake/CMakeLists.txt

Lines 185 to 188 in 436fcd2

	set(prefix ${CMAKE_INSTALL_PREFIX})
	set(exec_prefix "\${prefix}")
	set(libdir ${CMAKE_INSTALL_FULL_LIBDIR})
	set(includedir ${CMAKE_INSTALL_FULL_INCLUDEDIR})

Using just the absolute paths dos not make the package relocatable currently. See the linked libjxl and libtiff examples please, and the fix already merged in OCIO:

https://github.com/AcademySoftwareFoundation/OpenColorIO/blob/91761f256b18365b9b2f25fb0f083c592ea2628e/src/OpenColorIO/CMakeLists.txt#L179-L199

[kmilos]

Thanks, addressed by #1330