[devel] Cleanup of conditional compilation code and of background/gamma

handling. Internal changes only except for a new option to avoid compiling
in the png_build_grayscale_palette API (which is not used at all internally.)
The main change is to move the transform tests (READ_TRANSFORMS,
WRITE_TRANSFORMS) up one level to the caller of the APIs.
diff --git a/pngwtran.c b/pngwtran.c
index 2473d14..a5003d0 100644
--- a/pngwtran.c
+++ b/pngwtran.c
@@ -15,6 +15,7 @@
 
 #ifdef PNG_WRITE_SUPPORTED
 
+#ifdef PNG_WRITE_TRANSFORMS_SUPPORTED
 /* Transform the data according to the user's wishes.  The order of
  * transformations is significant.
  */
@@ -563,6 +564,7 @@
    }
 }
 #endif
+#endif /* PNG_WRITE_TRANSFORMS_SUPPORTED */
 
 #ifdef PNG_MNG_FEATURES_SUPPORTED
 /* Undoes intrapixel differencing  */