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

[alexshpilkin]

As per title: getdns.pc.in and getdns_ext_event.pc.in have

prefix=@prefix@
exec_prefix=${prefix}
libdir=${exec_prefix}/@CMAKE_INSTALL_LIBDIR@
includedir=${prefix}/@CMAKE_INSTALL_INCLUDEDIR@

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.

[wtoorop]

Reopened because current implementation depends on cmake_path which is not available on Ubuntu 20.04 (see #528). We need to include the module referenced in “Concatenating paths when building pkg-config files”.