pkg-config file is broken when CMAKE_INSTALL_{INCLUDE,LIB}DIR is absolute

[alexshpilkin]

As per title: CMakeLists.txt has

set (prefix ${CMAKE_INSTALL_PREFIX})
set (exec_prefix "\${prefix}")
set (libdir "\${exec_prefix}/${CMAKE_INSTALL_LIBDIR}")
set (includedir "\${prefix}/${CMAKE_INSTALL_INCLUDEDIR}")
set (bindir "${CMAKE_INSTALL_FULL_BINDIR}")
set (sysconfdir "${CMAKE_INSTALL_FULL_SYSCONFDIR}")
set (top_srcdir "${CMAKE_CURRENT_SOURCE_DIR}")

and so can’t handle absolute paths in CMAKE_INSTALL_{INCLUDE,LIB}DIR. This leads to broken .pc files on NixOS in particular.

See “Concatenating paths when building pkg-config files” for a thorough discussion of the problem and a suggested fix, or KDE’s extra-cmake-modules for a simpler approach. (Using FULL variables as already done for other directories is a possibility, but seems to be frowned upon by some other distributions. That applies just as much to BINDIR and SYSCONFDIR, though.)

[aberaud]

@alexshpilkin Thanks. What solution do you suggest ?
Ideally we would like to remain compatible with Ubuntu 18.04 (CMake 3.10)

[alexshpilkin]

@aberaud I’m not a CMake expert by any stretch of the imagination, but the KDE one, with if (IS_ABSOLUTE ...), seems to be compatible even with very old CMake versions, down to 3.0 even. It’s a bit less perfectionist than full path-joining in that it will leave a dot-dot component as is if someone passes one (why?..), but should still work correctly in all cases. (Even simpler would be to use the CMAKE_INSTALL_FULL_*DIR variables, which have also existed since basically forever, but this sacrifices relocatability, which some people apparently want.)

[n7tae]

Also, please add pkg-config to the build wiki in the Debian-Ubuntu section as not all popular versions have it.