[tlbuild] Strange build failures on Mac OS X 10.6

[Karl Berry]

      "_BrotliDecoderDecompress", referenced from:
          _sfnt_open_font in libfreetype.a(sfnt.o)

My only immediate information is that I have no "brotli" system library
on my centos7 build systems, yet of course the build works. There is no
mention of it in the build logs. Maybe there is mac-specific code in
libfreetype that is looking for brotli? Seems improbable.

brotli is included in texk/dvisvgm, but only for dvisvgm, because as far
as I knew nothing else needed it. Making it a TL-wide library would
.. not be fun. I doubt it is actually needed for anything we are doing.

Maybe some argument could be given to the freetype (and overall)
configure to not look for it? --without-brotli, from a glance at
freetype-src/builds/unix/configure ... probably won't be that easy ... -k