Fix pthread detection on Solaris 11.4 (#1479)

AX_PTHREAD requires _REENTRANT to confirm that pthread support is enabled.
Solaris 11.4 headers no longer use the macro, and GCC 16 therefore no
longer defines it for -pthread.

Detect XPG7 support in the target headers and require _REENTRANT only on
older Solaris releases. The existing pthread compile and link test remains
the final capability check.

This follows the canonical Autoconf Archive change:
https://github.com/autoconf-archive/autoconf-archive/pull/341

Regenerate configure with Autoconf 2.71.

Tested on Solaris 11.4 with GCC 15.2 and GCC 16.1. The Autoconf Archive
change was also tested on Solaris 11.3.

Co-authored-by: Rainer Orth <ro@CeBiTec.Uni-Bielefeld.DE>