cmake: Use CMAKE_INSTALL_FULL_* for libdir,bindir,etc.

This makes sure sdl2.pc gets the full paths, matching what the
configure script will generate.

Fixes #4569.

CMakeLists.txt

@@ -2348,9 +2348,9 @@ if(NOT WINDOWS OR CYGWIN OR MINGW)
   set(prefix ${CMAKE_INSTALL_PREFIX})
 
   set(exec_prefix "\${prefix}")
-  set(libdir "${CMAKE_INSTALL_LIBDIR}")
-  set(bindir "${CMAKE_INSTALL_BINDIR}")
-  set(includedir "${CMAKE_INSTALL_INCLUDEDIR}")
+  set(libdir "${CMAKE_INSTALL_FULL_LIBDIR}")
+  set(bindir "${CMAKE_INSTALL_FULL_BINDIR}")
+  set(includedir "${CMAKE_INSTALL_FULL_INCLUDEDIR}")
   if(SDL_STATIC)
     set(ENABLE_STATIC_TRUE "")
     set(ENABLE_STATIC_FALSE "#")