[libxml2] win32: allow passing *FLAGS on command line

[Nick Wellnhofer <nwellnhof src gnome org>]

commit a230b728f1289dd24c1666856ac4fb55579c6dfb
Author: Michael Stahl <Michael Stahl cib de>
Date:   Fri Apr 10 19:22:07 2020 +0200

    win32: allow passing *FLAGS on command line
    
    nmake is a primitive tool, so this is a primitive implementation:
    append EXTRA_CFLAGS etc. variables.
    
    Command line variables should be appended to allow overriding flags set
    in the makefile.
    
    It doesn't work to pass in CFLAGS like in make because that always
    overrides the assignments in the makefile.

 win32/Makefile.msvc | 5 +++++
 1 file changed, 5 insertions(+)
---
diff --git a/win32/Makefile.msvc b/win32/Makefile.msvc
index f6a0182d..725b0ca1 100644
--- a/win32/Makefile.msvc
+++ b/win32/Makefile.msvc
@@ -106,6 +106,11 @@ CFLAGS = $(CFLAGS) /D "NDEBUG" /O2
 LDFLAGS = $(LDFLAGS)
 !endif
 
+# append CFLAGS etc. passed on command line
+CPPFLAGS = $(CPPFLAGS) $(EXTRA_CPPFLAGS)
+CFLAGS = $(CFLAGS) $(EXTRA_CFLAGS)
+LDFLAGS = $(LDFLAGS) $(EXTRA_LDFLAGS)
+
 # Libxml object files.
 XML_OBJS = $(XML_INTDIR)\buf.obj\
        $(XML_INTDIR)\c14n.obj\