[libxslt] win32: allow passing *FLAGS on command line

[Nick Wellnhofer <nwellnhof src gnome org>]

commit 356246a928f48c1f1d4ac81904bf5194be0b9e97
Author: Nick Wellnhofer <wellnhofer aevum de>
Date:   Mon May 4 15:07:28 2020 +0200

    win32: allow passing *FLAGS on command line

 win32/Makefile.msvc | 5 +++++
 1 file changed, 5 insertions(+)
---
diff --git a/win32/Makefile.msvc b/win32/Makefile.msvc
index 2e4742bb..1f876684 100644
--- a/win32/Makefile.msvc
+++ b/win32/Makefile.msvc
@@ -73,6 +73,11 @@ LDFLAGS = $(LDFLAGS) /DEBUG
 CFLAGS = $(CFLAGS) /D "NDEBUG" /O2 
 !endif
 
+# append CFLAGS etc. passed on command line
+CPPFLAGS = $(CPPFLAGS) $(EXTRA_CPPFLAGS)
+CFLAGS = $(CFLAGS) $(EXTRA_CFLAGS)
+LDFLAGS = $(LDFLAGS) $(EXTRA_LDFLAGS)
+
 # Libxslt object files.
 XSLT_OBJS = $(XSLT_INTDIR)\attributes.obj\
        $(XSLT_INTDIR)\documents.obj\