pgsql: configure: More use of AC_ARG_VAR

configure: More use of AC_ARG_VAR

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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/29b3ac75462625b78eec0a92a49a22c5e2c7a01f

Modified Files
--------------
config/perl.m4     |  1 +
config/programs.m4 |  1 +
config/python.m4   |  1 +
config/tcl.m4      |  1 +
configure          | 16 +++++++++++++---
configure.in       |  1 +
6 files changed, 18 insertions(+), 3 deletions(-)