Install pkgconfig files from CMake installation

[metthal]

We were discussing this with Paul on gerrit here but to sum it up.

Right now, all popular Linux distributions package re2 from GNU make build which installs pkgconfig pc files. However this is not an ideal situation for people who use CMake in their projects. They can't do find_package(re2) in their project out of the box. The usual approach then is to write your own finder that will use pkgconfig to generate CMake targets but it's much better if projects provide their own. CMake build system in RE2 already generate all the necessary files for doing just find_package(re2) but it does not install pkgconfig files. That way, package maintainers for Fedora, Ubuntu, Arch, ... can't really use CMake and not break existing packages which rely on pkgconfig. I have provided patch for that but Paul asked me to write this issue to start the discussion regarding this matter.

There is also another thing which is that Red Hat based distributions should have 64-bit libraries installed in /usr/lib64 but through CMake, RE2 installs itself into /usr/lib. Therefore we also need GNUInstallDirs which work just find even on Windows and macOS.

cc @SuperWig

[SuperWig]

Well regarding pkgconfig, I'm not familar with it in the slightest so can't add input to that.

However this notification reminded me that I was working on getting the CMakeLists to be target based a couple months back.

SuperWig@79362c3

[junyer]

Thanks for filing the issue, @metthal, and thanks for responding, @SuperWig. The discussion that I anticipated hasn't eventuated, so let's move forward with the proposed change and see if someone complains later. :)

[junyer]

With GNU make, libre2.a and libre2.so are built. With CMake, libre2.a xor libre2.so is built. The difference is going to matter to package maintainers, presumably. Note also that the filename is baked into re2Config-release.cmake. :(

[junyer]

I don't mind keeping the GNUInstallDirs part of the change because handling lib versus lib64 et cetera is worthwhile, but if CMake doesn't support building libre2.a and libre2.so together and installing them together, then I think I'm going to have to undo the re2.pc part of the change.

[metthal]

I think it would be possible to somehow do that. I'm just kinda busy last several days. I'll probably message you one more time once I resolve simultaneous installation of static and dynamic library.

[junyer]

No worries. Just reopen this issue. :)