Fix build failure with --enable-quantify (bug 297430). r=bsmedberg, a=asa.

config/config.mk

@@ -261,7 +261,7 @@ endif
 ifdef MOZ_QUANTIFY
 # -FIXED:NO is needed for Quantify to work, but it increases the size
 # of executables, so only use it if building for Quantify.
-WIN32_EXE_LDFLAGS=-FIXED:NO
+WIN32_EXE_LDFLAGS += -FIXED:NO
 
 # We need -OPT:NOICF to prevent identical methods from being merged together.
 # Otherwise, Quantify doesn't know which method was actually called when it's