[tlbuild] Missing includes for zlib when building woff2 (dvisvgm)

[Karl Berry]

    mg> append $(ZLIB_INCLUDES) to AM_CFLAGS in file libs/ff-woff/Makefile.am?

I committed this to TL (r50610). Thanks.

    Maybe TeX Live could populate the ZLIB_CFLAGS variable? I don't know
    what's the most generic / standard way to specify flags in such cases.

There is no standard way. peb set everything up to use the
<lib>_INCLUDES and <lib>_LIBS variables. Somehow it would seem more
confusing than helpful to me to define both. -k