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

[Martin Gieseking]

Dear Mojca and Karl,

>> 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
> 
> The only other thing that would be nice would be to patch dvisvgm
> upstream to make this work automatically. I was just thinking aloud
> about what one could do in a reasonable way, since upstream doesn't
> know about TeX Live's own macros placed in top directories.

I'm going to add $(ZLIB_CFLAGS) to ffwoff/Makefile.am, and would prefer 
to keep using ZLIB_CFLAGS in the upstream sources because it matches the 
naming scheme of variables set by pkgconfig. It would be slightly 
inconsistent if I used _INCLUDE for zlib and _CFLAGS for libraries 
handled by pkgconfig. But since the affected Makefile.am is hardly ever 
changed, it shouldn't require too much additional work to adapt it in 
the TL sources.

Martin