Change default compiler for regional_Greece_small

In the script run_this_example.sh, the code was configured with

./configure F90=ifort MPIF90=/usr/local/openmpi-ifort/bin/mpif90 \
 FLAGS_CHECK="-O3 -assume byterecl" > tmp.log

by default. This is a not a standard setting. And it causes some issues
with running the buildbot test.

This commit sets the default to the GNU compilers.