pgsql: Rename configure.in to configure.ac

Rename configure.in to configure.ac

The new name has been preferred by Autoconf for a long time.  Future
versions of Autoconf will warn about the old name.

Discussion: https://www.postgresql.org/message-id/flat/e796c185-5ece-8569-248f-dd3799701be1%402ndquadrant.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/25244b8972a34b838c4033fe9efc1d31cba9d0e3

Modified Files
--------------
config/general.m4            |  2 +-
configure.in => configure.ac |  4 ++--
src/backend/catalog/Makefile |  4 ++--
src/include/pg_config.h.in   |  2 +-
src/tools/msvc/Solution.pm   | 10 +++++-----
src/tools/version_stamp.pl   | 12 ++++++------
6 files changed, 17 insertions(+), 17 deletions(-)