[libquadmath, patch, committed] PR55225: Add a configure check whether math.h has signgam

[Tobias Burnus]

POSIX's lgamma uses signgam – and hence has "extern int signgam" in math.h.

However, C99's lgamma doesn't mention signgam – and systems exist where 
math.h doesn't include it.

Solution: A configure check. Comitted as Rev. 193695.

Tobias
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