Improve -mpreferred-stack-boundary configure message.

Explicitly say we are checking for -mpreferred-stack-boundary=2.
Only 2 is ever used when it is supported. Some gcc versions/arches
(e.g. GCC 4.8 for x86_64) support -mpreferred-stack-boundary, but
only between 4 and 12. The message that -mpreferred-stack-boundary
wasn't supported was a little confusing. So explicitly say we checked
for -mpreferred-stack-boundary=2.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@14324 a5019735-40e9-0310-863c-91ae7b9d1cf9
Check some known PATHs for mpicc in configure.

On some distributions (fedora) mpicc not installed on the default PATH.
Add a search path for finding mpicc by default if it is installed.
The user can still override the used mpicc compiler using --with-mpicc=.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@14325 a5019735-40e9-0310-863c-91ae7b9d1cf9