Loosen restrictions on unrecognized compilers

A core tenet of CMake is the idea that you can use any valid C++
compiler. By enumerating all supported compilers and emitting and
hard error when an unrecognized compiler is detected, we are violating
that tenet.

Relaxing this message from a fatal error to merely a warning continues
to communicate to users that their build may not succeed but it leaves
the door open for the build to potential succeed if the compiler meets
all of our requirements.

cmake/Config.cmake

@@ -131,6 +131,5 @@ elseif(CMAKE_COMPILER_IS_GNUCXX)
         set(SFML_COMPILER_GCC_W64 1)
     endif()
 else()
-    message(FATAL_ERROR "Unsupported compiler")
+    message(WARNING "Unrecognized compiler: ${CMAKE_CXX_COMPILER_ID}. Use at your own risk.")
-    return()
 endif()