Avoid -Wexpansion-to-defined clang warning in wxOSX

Define wxCLRDLGG_USE_PREVIEW_WITH_ALPHA as either 0 or 1 instead of using an
expression involving "defined" for it as the latter provoked many clang
-Wexpansion-to-defined warnings, which were harmless but very annoying.

include/wx/generic/colrdlgg.h

@@ -21,9 +21,12 @@
 // Preview with opacity is possible only if wxGCDC and wxStaticBitmap are
 // available and currently it only works in wxOSX and wxMSW as it uses wxBitmap
 // UseAlpha() and HasAlpha() methods which only these ports provide.
-#define wxCLRDLGG_USE_PREVIEW_WITH_ALPHA \
-    ((wxUSE_GRAPHICS_CONTEXT && wxUSE_STATBMP) && \
+#if ((wxUSE_GRAPHICS_CONTEXT && wxUSE_STATBMP) && \
      (defined(__WXMSW__) || defined(__WXOSX__)))
+    #define wxCLRDLGG_USE_PREVIEW_WITH_ALPHA 1
+#else
+    #define wxCLRDLGG_USE_PREVIEW_WITH_ALPHA 0
+#endif
 
 #if wxCLRDLGG_USE_PREVIEW_WITH_ALPHA
 class wxStaticBitmap;