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

[alexshpilkin]

As per title: usrsctplib/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})

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

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