Make sdl2-config prefixes configurable again

The installation prefix ended up being hard-coded into the
`sdl2-config` script after the placeholders were accidentally removed
in 6956f4aa19.

CMakeLists.txt

@@ -2694,9 +2694,9 @@ if(NOT WINDOWS OR CYGWIN OR MINGW)
   set(prefix ${CMAKE_INSTALL_PREFIX})
 
   set(exec_prefix "\${prefix}")
-  set(libdir "${CMAKE_INSTALL_FULL_LIBDIR}")
-  set(bindir "${CMAKE_INSTALL_FULL_BINDIR}")
-  set(includedir "${CMAKE_INSTALL_FULL_INCLUDEDIR}")
+  set(libdir "\${exec_prefix}/${CMAKE_INSTALL_LIBDIR}")
+  set(bindir "\${exec_prefix}/${CMAKE_INSTALL_BINDIR}")
+  set(includedir "\${prefix}/${CMAKE_INSTALL_INCLUDEDIR}")
   if(SDL_STATIC)
     set(ENABLE_STATIC_TRUE "")
     set(ENABLE_STATIC_FALSE "#")