* m4/depend.m4 (AM_DEPENDENCIES): Add a definition to conftest.h,

* m4/depend.m4 (AM_DEPENDENCIES): Add a definition to conftest.h,
so as not to compile an empty source; some compilers fail on this.
Redirect stderr of depcc to /dev/null.  Test for -M with -o,
because some compilers will output the rules to the output file.
* depcomp (dashmstdout): Add --silent before --mode=compile, so
that we don't get libtool output mixed up with make rules.