pkg-config: fix double prefixed libdir= and includedir=

According to [1], CMAKE_INSTALL_<DIR> variables are allowed to be either
relative or absolute. This means one should never prefix to them, in
case they are already absolute. Use CMAKE_INSTALL_FULL_<DIR> instead, to
not end up with double prefix when the build system uses absolute paths.

This patch fixes broken rdma-core *.pc files in
https://github.com/nixos/nixpkgs.

[1] https://cmake.org/cmake/help/latest/module/GNUInstallDirs.html

Signed-off-by: Bjørn Forsman <bjorn.forsman@gmail.com>

buildlib/template.pc.in

@@ -1,7 +1,7 @@
 prefix=@CMAKE_INSTALL_PREFIX@
 exec_prefix=${prefix}
-libdir=${prefix}/@CMAKE_INSTALL_LIBDIR@
-includedir=${prefix}/@CMAKE_INSTALL_INCLUDEDIR@
+libdir=@CMAKE_INSTALL_FULL_LIBDIR@
+includedir=@CMAKE_INSTALL_FULL_INCLUDEDIR@
 
 Name: lib@PC_LIB_NAME@
 Description: RDMA Core Userspace Library