configure: search generic compilers for use in the tests

configure: search generic compilers for use in the tests

* configure.ac: Look for "generic" C, C++ and Fortran compilers,
with the aim of starting to use them in the testsuite (this will
be done in future changes).  This is more tricky than it seems,
since we don't want to abort the whole configure script even if
no one of those compilers is available (after all, they're only
needed by the testsuite, not to build automake), but currently
autoconf doesn't offer an easy way to obtain this behaviour.
We prefer non-GNU compilers to the GNU ones, to ensure better
coverage "in the wild".