Fix cmake path assumptions

[r-burns]

Fixes downstream packaging issue NixOS/nixpkgs#90610 (comment)

Paths from GNUInstallDirs are not necessarily relative, in which case
they can't be simply appended to the install prefix in the .pc file.
See https://github.com/jtojnar/cmake-snips#concatenating-paths-when-building-pkg-config-files

Also fixes hardcoded include directory.

[0-wiz-0]

Hm, then perhaps we should use CMAKE_INSTALL_FULL_<dir> in the pkgconfig file instead.

[r-burns]

I think it's preferred to have the .pc files interpolate relative to the installation prefix when applicable - cmake-snips mentions this briefly but doesn't go into the details of why.

[r-burns]

Ping, any more thoughts on this? If I understand correctly, this will work correctly on Windows too. Maybe I should remove or clarify the Windows not supported comment so it isn't misleading?

[0-wiz-0]

I think the correct solution is to change:

SET(bindir \${exec_prefix}/${CMAKE_INSTALL_BINDIR})
SET(libdir \${exec_prefix}/${CMAKE_INSTALL_LIBDIR})
SET(includedir \${prefix}/${CMAKE_INSTALL_INCLUDEDIR})

to

SET(bindir ${CMAKE_FULL_INSTALL_BINDIR})
SET(libdir ${CMAKE_FULL_INSTALL_LIBDIR})
SET(includedir ${CMAKE_FULL_INSTALL_INCLUDEDIR})

Does this work for you?

If not, can you please provide the full cmake invocation?

[r-burns]

Ok, I think that's a good call, the cmake is much simpler if just using the full path without interpolations is acceptable. Works for me, pushed changes.

[0-wiz-0]

Thanks!