cmake: get rid of the duplicated _USE_MATH_DEFINES

- define _USE_MATH_DEFINES in SDL_stdinc.h only
- define _USE_MATH_DEFINES if not defined already

CMakeLists.txt

@@ -999,7 +999,6 @@ if(SDL_LIBC)
     endforeach()
     set(HAVE_ALLOCA 1)
     check_symbol_exists(M_PI math.h HAVE_M_PI)
-    target_compile_definitions(sdl-build-options INTERFACE "-D_USE_MATH_DEFINES") # needed for M_PI
     set(STDC_HEADERS 1)
   else()
     set(HAVE_LIBC TRUE)

include/SDL_stdinc.h

@@ -80,7 +80,7 @@
 # include <ctype.h>
 #endif
 #ifdef HAVE_MATH_H
-# if defined(_MSC_VER)
+# if defined(_MSC_VER) && !defined(_USE_MATH_DEFINES)
 /* Defining _USE_MATH_DEFINES is required to get M_PI to be defined on
    Visual Studio.  See http://msdn.microsoft.com/en-us/library/4hwaceh6.aspx
    for more information.