pgsql: Fix AC_REQUIRES breakage in LLVM autoconf tests.

Fix AC_REQUIRES breakage in LLVM autoconf tests.

Any Autoconf macro that uses AC_REQUIRES -- directly or indirectly --
must not be inside a plain shell "if" test; if it is, whatever code
gets pulled in by the AC_REQUIRES will also be inside that "if".
Instead of "if" we can use AS_IF, which knows how to get this right
(cf commit 01051a987).

The only immediate problem from getting this wrong was that AC_PROG_AWK
had to be run twice, once inside the "if llvm" block and once in the
main line. However, it broke a different patch I'm about to submit
more thoroughly.

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/0b59b0e8bcc9e85bad6fffa7828344db1a36f20a

Modified Files
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configure | 46 ++--------------------------------------------
configure.in | 6 ++----
2 files changed, 4 insertions(+), 48 deletions(-)