sdl2-config.cmake.in: Use more CMake variables and less autotools ones

this makes it easier to create a portable sdl2-config.cmake that doesn't
hardcode its path (by replacing the hardcoded prefix with something
like "${CMAKE_CURRENT_LIST_DIR}/../../..")

sdl2-config.cmake.in

@@ -3,10 +3,11 @@
 set(prefix "@prefix@") 
 set(exec_prefix "@exec_prefix@")
 set(libdir "@libdir@")
-set(SDL2_PREFIX "@prefix@")
-set(SDL2_EXEC_PREFIX "@prefix@")
-set(SDL2_LIBDIR "@libdir@")
-set(SDL2_INCLUDE_DIRS "@includedir@/SDL2")
+set(includedir "@includedir@")
+set(SDL2_PREFIX "${prefix}")
+set(SDL2_EXEC_PREFIX "${exec_prefix}")
+set(SDL2_LIBDIR "${libdir}")
+set(SDL2_INCLUDE_DIRS "${includedir}/SDL2")
 set(SDL2_LIBRARIES "-L${SDL2_LIBDIR} @SDL_RLD_FLAGS@ @SDL_LIBS@")
 string(STRIP "${SDL2_LIBRARIES}" SDL2_LIBRARIES)
 
@@ -20,7 +21,7 @@ if(NOT TARGET SDL2::SDL2)
 
   add_library(SDL2::SDL2 SHARED IMPORTED)
   set_target_properties(SDL2::SDL2 PROPERTIES
-    INTERFACE_INCLUDE_DIRECTORIES "@includedir@/SDL2"
+    INTERFACE_INCLUDE_DIRECTORIES "${SDL2_INCLUDE_DIRS}"
     IMPORTED_LINK_INTERFACE_LANGUAGES "C"
     IMPORTED_LOCATION "@libdir@/${CMAKE_SHARED_LIBRARY_PREFIX}SDL2${CMAKE_SHARED_LIBRARY_SUFFIX}"
     # TODO: on windows set IMPORTED_IMPLIB to SDL2.lib
@@ -28,9 +29,9 @@ if(NOT TARGET SDL2::SDL2)
 
   add_library(SDL2::SDL2-static STATIC IMPORTED)
   set_target_properties(SDL2::SDL2-static PROPERTIES
-    INTERFACE_INCLUDE_DIRECTORIES "@includedir@/SDL2"
+    INTERFACE_INCLUDE_DIRECTORIES "${SDL2_INCLUDE_DIRS}"
     IMPORTED_LINK_INTERFACE_LANGUAGES "C"
-    IMPORTED_LOCATION "@libdir@/${CMAKE_STATIC_LIBRARY_PREFIX}SDL2${CMAKE_STATIC_LIBRARY_SUFFIX}"
+    IMPORTED_LOCATION "${SDL2_LIBDIR}/${CMAKE_STATIC_LIBRARY_PREFIX}SDL2${CMAKE_STATIC_LIBRARY_SUFFIX}"
     INTERFACE_LINK_LIBRARIES "${SDL2_EXTRA_LINK_FLAGS_STATIC}")
 
   add_library(SDL2::SDL2main STATIC IMPORTED)