Bug 207421: use /Op to disable some floating-point optimizations of MSVC

6.0 Processor Pack because they make the floating-point calculations in
prdtoa.c produce incorrect results.  r=cls.

nsprpub/pr/src/misc/Makefile.in

@@ -82,6 +82,16 @@ RELEASE_BINS = $(srcdir)/compile-et.pl $(srcdir)/prerr.properties
 
 include $(topsrcdir)/config/rules.mk
 
+# Prevent floating point errors caused by MSVC 6.0 Processor Pack
+# optimizations (bug 207421).  This disables optimizations that
+# could change the precision of floating-point calculations for
+# this single compilation unit.
+ifeq ($(NS_USE_GCC)_$(OS_ARCH),_WINNT)
+$(OBJDIR)/prdtoa.$(OBJ_SUFFIX): prdtoa.c
+	@$(MAKE_OBJDIR)
+	$(CC) -Fo$@ -c $(CFLAGS) -Op $(call abspath,$<)
+endif
+
 #
 # Generate prerr.h, prerr.c, and prerr.properties from prerr.et.
 #