superenv: fix CC value for ENV.gcc

'gcc' could refer to anything of llvm-gcc, clang, or gcc-4.0 on
various Xcode versions.
gstreamer & gst-plugins: --HEAD

Building from gstreamer master is required for development
purpose.

Also gest-plugins-* and gst-libav get --HEAD.

Closes #21665.

Signed-off-by: Samuel John <github@SamuelJohn.de>
gobject-introspection: depends_on :python

To avoid ImportError: Symbol not found: _PyList_Check
during build.
gst-plugins-base: --with-gobject-introspection

should propagate to gstreamer otherwise build will fail.
xmto: fix missing DTD/entities errors

`xmlto` forces the `--nonet` flag on `xsltproc`, which means it throws up an
I/O error whenever it needs to hit the network to pull down any files. So we
patch out the forced flag, and additionally add `docbook-xsl` as a dependency,
which'll reduce the odds of having to hit the network in the first place.

Fixes #21040.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>
kytea 0.4.6

Closes #22089.

Signed-off-by: Adam Vandenberg <flangy@gmail.com>
zsh completion for brew reinstall

Closes #22061.

Signed-off-by: Samuel John <github@SamuelJohn.de>