Commit 9fa6f00b assumed that __builtin_constant_p("string literal")
is TRUE, if the compiler has that function at all.  Buildfarm results
show that Sun Studio 12, at least, breaks that assumption.  Removing
that usage would leave us with no mechanical check for a very fragile
coding requirement, so instead teach configure to ignore
__builtin_constant_p() if it doesn't behave that way.  We could
complicate matters by distinguishing three cases (no such function,
vs does, vs doesn't work for string literals); but for now, that seems
unnecessary because our other existing uses of this function are just
fairly minor optimizations of non-returning elog/ereport.  We can live
without that on the small population of compilers that act this way.

Discussion: https://postgr.es/m/22997.1513264066@sss.pgh.pa.us