cmake: make the .pc file relocatable

[lazka]

Instead of writing absolute file paths for exec_prefix, libdir and
includedir make them all relative to prefix. This makes the .pc
file match the autotools generated one and allows changing all paths
via prefix.

Before:

prefix=C:/test
exec_prefix=C:/test
libdir=C:/test/lib
includedir=C:/test/include

After:

prefix=C:/test
exec_prefix=${prefix}
libdir=${exec_prefix}/lib
includedir=${prefix}/include

While CMAKE_INSTALL_LIBDIR and CMAKE_INSTALL_INCLUDEDIR are by default
relative paths, cmake allows users to override them with absolute ones,
so in case they are absolute write them as is into the .pc file and
not relative to the prefix.

[lazka]

(someone mentioned CMAKE_INSTALL_LIBDIR can be absolute. I'll update this PR to handle that case)

[lazka]

(someone mentioned CMAKE_INSTALL_LIBDIR can be absolute. I'll update this PR to handle that case)

done now, better late then never :)