Avoid clang 4.0 -Wexpansion-to-defined warnings

It's not really clear why, but clang 4.0 has decided to start giving warnings
about using the result of an expression constructed using the preprocessor
"defined" operation in #if checks, so trivially avoid doing this.

src/unix/utilsunx.cpp

@@ -20,7 +20,11 @@
 
 #include "wx/utils.h"
 
-#define USE_PUTENV (!defined(HAVE_SETENV) && defined(HAVE_PUTENV))
+#if !defined(HAVE_SETENV) && defined(HAVE_PUTENV)
+    #define USE_PUTENV 1
+#else
+    #define USE_PUTENV 0
+#endif
 
 #ifndef WX_PRECOMP
     #include "wx/string.h"