textproc/{,py-}libxml2: switch back to autotools

See notes in Makefile and linked PRs.

While here, disable the ICU option by default. CFLAGS for libicu
are passed to libxml2's consumers when the option is enabled. As
icu's API is not stable between versions, PORTREVISION bumps can
get missed.

Additionally, properly exclude all OPTIONS from py-libxml2 and
prevent do-configure from unnecessarily running.

Co-authored-by: diizzy
PR: 262853, 262940, 262877
Approved by: fluffy (mentor)