pgsql: Explicitly require MIT Kerberos for GSSAPI

Explicitly require MIT Kerberos for GSSAPI

WHen building with GSSAPI support, explicitly require MIT Kerberos and
check for gssapi_ext.h in configure.ac and meson.build.  Also add
documentation explicitly stating that we now require MIT Kerberos when
building with GSSAPI support.

Reveiwed by: Johnathan Katz
Discussion: https://postgr.es/m/abcc73d0-acf7-6896-e0dc-f5bc12a61bb1@postgresql.org

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/f7431bca8b0138bdbce7025871560d39119565a0

Modified Files
--------------
configure                            | 27 +++++++++++++++++++++++++++
configure.ac                         |  2 ++
doc/src/sgml/client-auth.sgml        |  2 +-
doc/src/sgml/installation.sgml       | 21 +++++++++++----------
meson.build                          | 10 ++++++++++
src/backend/libpq/auth.c             |  5 +++--
src/backend/libpq/be-secure-gssapi.c |  5 +++--
7 files changed, 57 insertions(+), 15 deletions(-)