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

[alexshpilkin]

As per title: CMakeLists.txt has

  set(prefix ${CMAKE_INSTALL_PREFIX})
  set(exec_prefix "\$\{prefix\}")
  set(libdir "\$\{prefix\}/${CMAKE_INSTALL_LIBDIR}")
  set(includedir "\$\{prefix\}/${CMAKE_INSTALL_INCLUDEDIR}")

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

Identical to Matroska-Org/libebml#97. Similar to open-source-parsers/jsoncpp#1199. See “Concatenating paths when building pkg-config files” for a discussion of the problem in a somewhat different context and a suggested fix (I don’t know CMake myself, sorry).