pgsql: Postpone LLVM-related uses of AC_CHECK_DECLS.

Postpone LLVM-related uses of AC_CHECK_DECLS.

Calling AC_CHECK_DECLS before we've finished setting up the compiler's
CFLAGS seems like a pretty risky proposition, especially now that the
first use of that macro will result in a test to see whether the compiler
gives warning or error for undeclared built-in functions.  That answer
could very easily get changed later than where PGAC_LLVM_SUPPORT is
called; furthermore, it's hardly unlikely that flags such as -D_GNU_SOURCE
could change visibility of declarations.  Hence, be a little less cavalier
about where to do LLVM-related tests.  This results in v11 and HEAD doing
the warning-or-error check at the same place in the script as older
branches are doing it, which seems like a good thing.

Per further thought about commits 0b59b0e8b and 16fbac39f.

Branch
------
REL_11_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/923f9c2def111e65a83f65d7caf75efb772314fc

Modified Files
--------------
config/llvm.m4 |  31 +++--
configure      | 406 +++++++++++++++++++++++++++++----------------------------
configure.in   |   4 +
3 files changed, 230 insertions(+), 211 deletions(-)