CMake: fix installation of proj.pc on Windows (fixes #3078)

[rouault]

No description provided.

[rouault]

hum, from the log of a build before the change (https://github.com/OSGeo/PROJ/runs/5470513722?check_suite_focus=true), proj.pc was correctly installed in D:/a/PROJ/PROJ/proj_dir/lib/pkgconfig/proj.pc

@shawnlaffan could you confirm if this pull request improves things for you

But I notice there's another issue on Windows with man pages being installed in /man1/ ( although man/CMakeLists.txt has DESTINATION share/man/man1)

[shawnlaffan]

Thanks @rouault

This fixes the pkgconfig issue I reported in #3078

[shawnlaffan]

And FWIW, the man1 dir for my builds is at .../share/man/man1, not at the system root level.

[dg0yt]

I would prefer if GNUInstallDirs would be used consistently. It is the only common denominator for configuring packages in the CMake world. (It also facilitates contributions.)

[rouault]

I would prefer if GNUInstallDirs would be used consistently.

agreed, but too substantial to attack for a bug fix. I've filed that as #3100