CMake: add support for pkg-config

[mwtoews]

This addresses part of #2546 to install a pkg-config file.

[mwtoews]

Does anyone feel qualified to review this? Nevertheless, the CMake-generated set-up passes post-install tests with Linux and macOS.

One observation discovered here between autotools vs CMake installs on macOS is that the dynamic library via CMake has the relative reference @rpath/libproj.22.dylib (via otool -L) whereas the autotools library uses an absolute path '/tmp/proj_cmake_install/lib/libproj.22.dylib`. This shouldn't be surprising given the CMake options, but is a difference to note.

[rouault]

not a cmake expert, but that looks very reasonable to me

[dg0yt]

Given the syntax of the root proj.pc.in, did you conside creating a function (i.e. scope) that sets the variables needed to configure that file, instead of introducing a separate cmake/proj.pc.cmake.in? But changes should be rare, so redundancy may be acceptable.

[dg0yt]

I guess Debian will be unhappy about this rule. Due to multi-arch, proj.pc resides in /usr/lib/x86_64-linux-gnu/pkgconfig/. In a GNUInstallDirs context, the destination would probably be ${CMAKE_INSTALL_LIBDIR}/pkgconfig.

[kbevers]

Is everyone happy with this PR in its current state?

[dg0yt]

I just had nothing but MinGW (MSYS2) today, and guess what... There are other pkg-config files here. I successfully tested build/install with:

if(UNIX OR MINGW)

[mwtoews]

hmm, possibly this one too, as some of these paths are set from GNUInstallDirs

PROJ/cmake/ProjInstallPath.cmake

Lines 26 to 27 in 1a51877

	if(UNIX)
	include(GNUInstallDirs)

[dg0yt]

I see. I don't see a difference in proj.pc when I change ProjInstallPath.cmake.
And I wouldn't delay PROJ 8.0.0 for MINGW pkgconfig.

Default config for MinGW, generator Ninja:

prefix=c:/OSGeo4W
exec_prefix=${prefix}
libdir=${exec_prefix}/lib
includedir=${prefix}/include
datarootdir=${prefix}/share
datadir=${prefix}/share/proj

Name: PROJ
Description: Coordinate transformation software library
Requires:
Version: 8.0.0
Libs: -L${libdir} -lproj
Libs.private: -lsqlite3 -ltiff -lcurl -lstdc++
Cflags: -I${includedir}

Looks good to me.

[mwtoews]

Given the syntax of the root proj.pc.in, did you conside creating a function (i.e. scope) that sets the variables needed to configure that file

Sure, I'll give this a "go"...

[dg0yt]

LGTM.

$ diff former-mingw current-mingw
6c6
< datadir=${prefix}/share/proj
---
> datadir=${datarootdir}/proj

[mwtoews]

This last update, which uses the same proj.pc.in file as by autotools, creates an identical proj.pc file for a given prefix.