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

@@ -13,6 +13,7 @@ set(CMAKE_DEBUG_POSTFIX "d" CACHE STRING "Override CMAKE_DEBUG_POSTFIX.")
 list(APPEND CMAKE_MODULE_PATH ${CMAKE_CURRENT_SOURCE_DIR}/builds/cmake/Modules)
 
 include(ConfigureWindows)
+include(GNUInstallDirs)
 
 # Doxygen
 find_package(Doxygen)
@@ -278,8 +279,8 @@ 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)