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.

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