Fix pkgconfig files install path

[texus]

Description

When running CMake without setting CMAKE_INSTALL_PREFIX, and then running CMake again with a different prefix, the install path for the pkgconfig files still points to the old location.

Because the default /usr/local prefix on Linux requires root access, installing SFML without sudo will fail with an error like this:

-- Install configuration: "Release"
-- Installing: /home/texus/Documents/SFML/whatever/lib/libsfml-system-s.a
-- Installing: /home/texus/Documents/SFML/whatever/lib/libsfml-window-s.a
-- Installing: /home/texus/Documents/SFML/whatever/lib/libsfml-network-s.a
-- Installing: /home/texus/Documents/SFML/whatever/lib/libsfml-graphics-s.a
-- Installing: /home/texus/Documents/SFML/whatever/lib/libsfml-audio-s.a
-- Installing: /usr/local/lib/pkgconfig/sfml-all.pc
CMake Error at build/cmake_install.cmake:59 (file):
  file INSTALL cannot copy file
  "/home/texus/Documents/SFML/build/tools/pkg-config/sfml-all.pc" to
  "/usr/local/lib/pkgconfig/sfml-all.pc": Permission denied.

This was fixed in the CSFML project a few weeks ago (see SFML/CSFML#233 and SFML/CSFML#234), this PR applies the same fix to SFML.

This change is NOT backwards compatible. The existing SFML_PKGCONFIG_INSTALL_PREFIX variable will no longer work and is being replaced with a new SFML_PKGCONFIG_INSTALL_DIR variable in cmake.

Tasks

• Tested on Linux

How to test this PR?

On linux you can run the following commands to get the error:

cmake -S . -B build
cmake --build build
cmake --install build --prefix whatever

On other platforms you can probably also witness the files being installed to the wrong directory, but you will need to set SFML_INSTALL_PKGCONFIG_FILES to ON yourself because it is only enabled by default on Linux and BSD.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 44.13%. Comparing base (6eaf300) to head (e0b6879).
Report is 44 commits behind head on master.

❗ Current head e0b6879 differs from pull request most recent head e4c2d0a. Consider uploading reports for the commit e4c2d0a to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2917      +/-   ##
==========================================
+ Coverage   43.65%   44.13%   +0.47%     
==========================================
  Files         253      253              
  Lines       20940    21269     +329     
  Branches     5139     5210      +71     
==========================================
+ Hits         9142     9387     +245     
- Misses      10785    11057     +272     
+ Partials     1013      825     -188     

see 74 files with indirect coverage changes

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[coveralls]

Pull Request Test Coverage Report for Build 9074962350

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 51.9%

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Totals
Change from base Build 9057848497:	0.0%
Covered Lines:	10476
Relevant Lines:	19004

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💛 - Coveralls