depend: assume '-c' compiler flag always means to produce object files

depend: assume '-c' compiler flag always means to produce object files

Rather than fully-fledged executables.  This is the case for all the
compilers of all languages supported by Automake.

This is just a simplification; no semantic changes are intended.

* bin/automake.in (handle_languages): Drop transform '%-c%'.
* lib/am/depend2.am: Adjust to just assume the '%-c%' transform
expands to "-c".

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