[bugfix,mpi-config] Corrected path to installed m4 files.

Previously running the installed mpi-config failed as mpi-config.m4
was not found. This patch corrects the path to it in the script and
fixes the problem.
(cherry picked from commit b1dc9a162b040a6096bc9ec5fe99d0b4e7d97307)

Signed-off-by: Markus Blatt <markus@dr-blatt.de>