xml2: switch from autotools to meson

[kleisauke]

Before:

-rw-r--r-- 1 root root   878202 Jul  8 09:21 libxml2.a

After

-rw-r--r-- 1 root root   554918 Jul  8 11:34 libxml2.a

Marked as draft as I didn't test this at runtime yet. I suspect the reader, writer, valid, http and zlib features of libxml2 were required by libgsf instead of librsvg.

[kleisauke]

Seems to work! I tested this with:

$ LD_PRELOAD=$HOME/libvips-8.15.2-linux-x64/lib/libvips-cpp.so.42 npm run test
[...]
  1199 passing (4s)

on sharp's test suite.

[lovell]

Reduced binary size and reduced attack surface, excellent, thank you Kleis.