gdb: use AC_PROG_CC_STDC instead of AM_PROG_CC_STDC

gdb: use AC_PROG_CC_STDC instead of AM_PROG_CC_STDC

`autoconf -Wall` notes that AM_PROG_CC_STDC is obsolete:
Fixes this autoconf warning:

    configure.ac:40: warning: 'AM_PROG_CC_STDC': this macro is obsolete.
    configure.ac:40: You should simply use the 'AC_PROG_CC' macro instead.
    configure.ac:40: Also, your code should no longer depend upon 'am_cv_prog_cc_stdc',
    configure.ac:40: but upon 'ac_cv_prog_cc_stdc'.
    aclocal.m4:770: AM_PROG_CC_STDC is expanded from...
    configure.ac:40: the top level

Since we build with a C++ compiler now, I don't think this is relevant.
If you look at the messages removed from gdbsupport/aclocal.m4, it says
that this functionality is now integrated in AC_PROG_CC, which we
already call.  So it might not even make a difference.

We had a local version of AM_PROG_CC_STDC, in gdb/acinclude.m4 (only
used by gdb/configure.ac), remove it.

gdb/ChangeLog:

	* acinclude.m4 (AM_PROG_CC_STDC): Remove.
	* configure: Re-generate.
	* configure.ac: Remove AM_PROG_CC_STDC.

gdbsupport/ChangeLog:

	* aclocal.m4: Re-generate.
	* configure: Re-generate.
	* configure.ac: Remove AM_PROG_CC_STDC.

Change-Id: Ic824393598805d4f78cda9d119f8af46096e9c73