flann 1.8.2

Closes #16724.
exempi: note that patch has been merged upstream
pxz: requires openmp, not available in clang

Closes #16227.
superenv: favor /opt/X11 over /usr/X11 when both exist

This is the order of precedence used elsewhere in Homebrew when
detecting X11 versions and prefixes.

Fixes #16598.
GnuTLS2 2.12.21

Adding GnuTLS 2.x
GnuTLS 3.x changed the API slightly, but also changes license terms,
so it is unlikely that some software will update to newer versions.

Closes #17109.
Wireshark: link against GnuTLS 2

Closes #16257.
mutt: remove unused flag