AC_PROG_CC: also check whether $CC supports "-c -o" together

AC_PROG_CC: also check whether $CC supports "-c -o" together

This is for Automake and its 'subdir-object' mode (see automake bug#13378,
in particular <http://debbugs.gnu.org/cgi/bugreport.cgi?bug=13378#73>), so
we make the new behavior available only if the special witness macro
'_AM_PROG_CC_C_O_HELPME' is defined (future Automake versions will define
it).  We might decide, at a later date, to make this behaviour public;
but then we'll have to discuss its usefulness and design more in depth,
and if they are agreed upon, adjust the documentation to match, and also
decide what to do with the macro AC_PROG_CC_C_O, with its similar (but
slightly incompatible) semantics; since doing so right now would bring us
off-track (and Automake needs this change *today*, or better, yesterday),
we proceed with this simpler hack.

* lib/autoconf/c.m4 (AC_PROG_CC): Adjust.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>