[proj4] Install pc file

[dg0yt]

• What does your PR fix?

  Installs the pkgconfig file for proj.
  This is missing only in the cmake build system of PROJ 7.2.1.
  This is a prerequisite for improving the port's feature interface without breaking depending ports such as libspatialite (cf. [proj4] Update to 8.1.1, revise features and dependencies #20443, [libspatialite] Use pkgconfig for nmake and autotools #20480).

• Which triplets are supported/not supported? Have you updated the CI baseline?

  all (unchanged), no

• Does your PR follow the maintainer guide?

  yes

• If you have added/updated a port: Have you run ./vcpkg x-add-version --all and committed the result?

  yes

[dg0yt]

@strega-nil-ms This change uses VCPKG_SYSTEM_LIBRARIES from vcpkg_common_definitions.cmake to determine the C++ standard library.
However, this variable doesn't have the literal name (stdc++ etc.) but escapes + (stdc\+\+). Is this intentional?
(I also don't see how a triplet could override the C++ library e.g. for clang toolchains instead of g++ toolchains.)

[strega-nil-ms]

I need time to review, so assigning to myself.

[strega-nil-ms]

After discussion, we've agreed that VCPKG_SYSTEM_LIBRARIES is not used anywhere and therefore we should just remove it entirely; the correct way to check for these things is to ask CMake which libraries it needs to link to privately. @ras0219 is currently trying to figure out how to query CMake for these things.

[dg0yt]

After discussion, we've agreed that VCPKG_SYSTEM_LIBRARIES is not used anywhere and therefore we should just remove it entirely; the correct way to check for these things is to ask CMake which libraries it needs to link to privately. @ras0219 is currently trying to figure out how to query CMake for these things.

Thanks for feedback. Well, removal was the most likely outcome that I expected when I started using it.
I wonder if you should deprecate the variable in one "release" before removing it before removing it in another, if you care for user-side overlay ports and registries. It is not used anywhere in vcpkg, but it is not an internal feature.

[dg0yt]

@ras0219 is currently trying to figure out how to query CMake for these things.

I don't want to delay this PR. I use the same condition now as port gdal does for linking proj.

[JackBoosY]

Ping @strega-nil-ms for response.

[dg0yt]

FTR I did the experimentation in #20480. This is the result from the lesson-learnt.