Make SCons's DEBUG/_DEBUG consistent with CMake

Also move this to a generic place in SConstruct instead of having this
defines in 6 different configurations.

Should not be functional changes, but please verify all the platforms.

SConstruct

@@ -210,6 +210,11 @@ if sys.platform=='win32':
     if B.bitness==64:
         env.Append(CPPFLAGS=['-DWIN64']) # -DWIN32 needed too, as it's used all over to target Windows generally
 
+if env['BF_DEBUG']:
+    env.Append(CPPDEFINES=['_DEBUG', 'DEBUG'])
+else:
+    env.Append(CPPDEFINES=['NDEBUG'])
+
 if not env['BF_FANCY']:
     B.bc.disable()