re PR bootstrap/50461 (mpfr.h found in mpfr-3.1.0/src instead of mpfr-3.0.1/. as previously)

	PR bootstrap/50461
	* configure.ac (mpfr-dir): When using in-tree MPFR sources
	allow for the fact that from release v3.1.0 of MPFR the source
	files were moved into a src sub-directory.
	* configure: Regenerate.

Co-Authored-By: Paul Smith <psmith@gnu.org>

From-SVN: r187341