build: depend on Automake >= 1.14

Automake 1.14 has been released 3 years ago, so I think we can depend on
it.

This permits to remove the call to AM_PROG_CC_C_O.

From the Automake documentation:

https://www.gnu.org/software/automake/manual/automake.html

AM_PROG_CC_C_O

    This is an obsolescent macro that checks that the C compiler
supports the -c and -o options together. Note that, since Automake 1.14,
the AC_PROG_CC is rewritten to implement such checks itself, and thus
the explicit use of AM_PROG_CC_C_O should no longer be required.

https://bugzilla.gnome.org/show_bug.cgi?id=773900