canvas horkage fix

content/canvas/src/Makefile.in

@@ -75,7 +75,12 @@ REQUIRES	= \
 		plugin \
 		$(NULL)
 
-CPPSRCS		= \
+# XXX some platforms can't handle building
+# an empty .a/lib.  Remove this dummy.cpp
+# whenever w have a rendering context
+# that doesn't depend on any non-default
+# libraries.
+CPPSRCS		= dummy.cpp \
 		$(NULL)
 
 ifdef MOZ_ENABLE_CAIRO

content/canvas/src/dummy.cpp

@@ -0,0 +1,4 @@
+/*
+ * This file only exists because some platforms can't handle building an empty .a
+ * file, or .lib, or whatever (e.g. OS X, possibly Win32).
+ */