Installed Clipper2.pc:: Use canonical way to refer to inc and libdir

Cmake already provides variables where the installed libraries
and include directories end up, so use these for most compatible
way of generating the package config.

CPP/Clipper2.pc.cmakein

@@ -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: Clipper2@PCFILE_LIB_SUFFIX@
 Description: A Polygon Clipping and Offsetting library in C++