cmake: file(RELATIVE_PATH) needs 2 absolute paths

cmake/sdltargets.cmake

@@ -332,6 +332,9 @@ function(configure_sdl3_pc)
   endif()
 
   # Calculate prefix relative to location of sdl3.pc
+  if(NOT IS_ABSOLUTE "${CMAKE_INSTALL_PREFIX}")
+    set(CMAKE_INSTALL_PREFIX "${CMAKE_BINARY_DIR}/${CMAKE_INSTALL_PREFIX}")
+  endif()
   file(RELATIVE_PATH SDL_PATH_PREFIX_RELATIVE_TO_PKGCONFIG "${CMAKE_INSTALL_PREFIX}/${SDL_PKGCONFIG_INSTALLDIR}" "${CMAKE_INSTALL_PREFIX}")
   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}")