AC_ARG_VAR is necessary if an environment variable influences a
configure result that is then used by other tests that are cached.
With AC_ARG_VAR, a change in the variable is detected on subsequent
configure runs and the user is then advised to remove the cache.

This adds AC_ARG_VAR calls for: MSGFMT, PERL, PYTHON, TCLSH, XML2_CONFIG
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://www.postgresql.org/message-id/30672.1546816567@sss.pgh.pa.us