cmake: don't prepend absolute CMAKE_INSTALL_{INCLUDE,LIB}DIR with ${prefix}

cmake/sdl3.pc.in

@@ -1,7 +1,7 @@
 prefix=@SDL_PKGCONFIG_PREFIX@
 exec_prefix=${prefix}
-libdir=${prefix}/@CMAKE_INSTALL_LIBDIR@
-includedir=${prefix}/@CMAKE_INSTALL_INCLUDEDIR@
+libdir=@LIBDIR_FOR_PKG_CONFIG@
+includedir=@INCLUDEDIR_FOR_PKG_CONFIG@
 
 Name: sdl3
 Description: Simple DirectMedia Layer is a cross-platform multimedia library designed to provide low level access to audio, keyboard, mouse, joystick, 3D hardware via OpenGL, and 2D video framebuffer.

cmake/sdltargets.cmake

@@ -346,6 +346,17 @@ function(configure_sdl3_pc)
   string(REGEX REPLACE "[/]+$" "" SDL_PATH_PREFIX_RELATIVE_TO_PKGCONFIG "${SDL_PATH_PREFIX_RELATIVE_TO_PKGCONFIG}")
   set(SDL_PKGCONFIG_PREFIX "\${pcfiledir}/${SDL_PATH_PREFIX_RELATIVE_TO_PKGCONFIG}")
 
+  if(IS_ABSOLUTE "${CMAKE_INSTALL_INCLUDEDIR}")
+    set(INCLUDEDIR_FOR_PKG_CONFIG "${CMAKE_INSTALL_INCLUDEDIR}")
+  else()
+    set(INCLUDEDIR_FOR_PKG_CONFIG "\${prefix}/${CMAKE_INSTALL_INCLUDEDIR}")
+  endif()
+  if(IS_ABSOLUTE "${CMAKE_INSTALL_LIBDIR}")
+    set(LIBDIR_FOR_PKG_CONFIG "${CMAKE_INSTALL_LIBDIR}")
+  else()
+    set(LIBDIR_FOR_PKG_CONFIG "\${prefix}/${CMAKE_INSTALL_LIBDIR}")
+  endif()
+
   configure_file("${SDL3_SOURCE_DIR}/cmake/sdl3.pc.in" "${SDL3_BINARY_DIR}/sdl3.pc" @ONLY)
 endfunction()