cmake: fall back to -Wconversion if -Wfloat-conversion isn't available

... during checks for valid isinf/isinff/isnan/isnanf macros.

CMakeLists.txt

@@ -1085,8 +1085,10 @@ if(SDL_LIBC)
   cmake_push_check_state()
   if(MSVC)
     string(APPEND CMAKE_REQUIRED_FLAGS " -we4244 -WX")  # 'conversion' conversion from 'type1' to 'type2', possible loss of data
-  else()
+  elseif(HAVE_GCC_WFLOAT_CONVERSION)
     string(APPEND CMAKE_REQUIRED_FLAGS " -Wfloat-conversion -Werror")
+  else()
+    string(APPEND CMAKE_REQUIRED_FLAGS " -Wconversion -Werror")
   endif()
   foreach(math_fn isinf isnan)
     string(TOUPPER "${math_fn}" MATH_FN)