pkgconfig: Fix support for absolute lib and include dirs

Using GNUInstallDirs to always have an absolute path regardless of the
user input: https://cmake.org/cmake/help/latest/module/GNUInstallDirs.html

CMakeLists.txt

@@ -273,22 +273,22 @@ endif()
 # .so version
 set_property(TARGET lcf PROPERTY SOVERSION 0)
 
+# installation
+include(GNUInstallDirs)
+
 # pkg-config file generation
 set(PACKAGE_TARNAME ${PROJECT_NAME})
 set(PACKAGE_VERSION ${PROJECT_VERSION})
 set(prefix "${CMAKE_INSTALL_PREFIX}")
 set(exec_prefix "\${prefix}")
-set(libdir "\${exec_prefix}/${CMAKE_INSTALL_LIBDIR}")
-set(includedir "\${prefix}/${CMAKE_INSTALL_INCLUDEDIR}")
+set(libdir "${CMAKE_INSTALL_FULL_LIBDIR}")
+set(includedir "${CMAKE_INSTALL_FULL_INCLUDEDIR}")
 string(REPLACE ";" " " AX_PACKAGE_REQUIRES_PRIVATE "${LIBLCF_DEPS}")
 configure_file(builds/${PROJECT_NAME}.pc.in builds/${PROJECT_NAME}.pc @ONLY)
 
 # Cmake-config file generation
 configure_file(builds/${PROJECT_NAME}-config.cmake.in builds/${PROJECT_NAME}-config.cmake @ONLY)
 
-# installation
-include(GNUInstallDirs)
-
 install(
 	TARGETS lcf
 	ARCHIVE DESTINATION ${CMAKE_INSTALL_LIBDIR}