Conversion to autoconf:

Conversion to autoconf:
* acconfig.h, aclocal.m4: New files.
* configure.in: Rewritten (except for some target-specific code) for autoconf.
* conf.in, configure: New files, generated from the above.
* Makefile.in: Changed magic sequence indicating insertion of makefile
fragments.
(VPATH, srcdir, CC, LIBS, OBJS dependencies): Use @-substitutions from
configure.
(LINKED_HEADERS): Deleted a.out.gnu.h, a.out.h, and host.h.
(config.status, configure): Rewrite rules.
(config-stamp): Depend on conf.  Skip variables that configure is now
substituting itself.
(*.o dependencies): Deleted host.h.
(distclean, realclean): Don't delete host.h.
* as.c: Don't include stdio.h, string.h, sys/types.h.  Include signal.h after
as.h.
* as.h: Include alloca-conf.h first.  Include ctype.h, string.h, strings.h,
stdlib.h, unistd.h, sys/types.h, fopen-bin.h, fopen-same.h, as suggested by
autoconf test results.
[BROKEN_ASSERT]: Don't include assert.h.
(strdup): Declare.
(volatile, const): Define if not __STDC__ and not already defined.
(malloc, realloc) [NEED_MALLOC_DECLARATION]: Declare.
(free) [NEED_FREE_DECLARATION]: Declare.
* gasp.c: Include config.h, stdlib.h (if HAVE_STDLIB_H).  Don't include host.h.
(malloc) [NEED_MALLOC_DECLARATION]: Declare.
* messages.c: Include as.h first.  Include errno.h only if HAVE_ERRNO_H.  If
HAVE_VARARGS_H and not __STDC__, undefine HAVE_STDARG_H.  Set NO_STDARG and
NO_VARARGS as appropriate.
* doc/Makefile.in (srcdir, INSTALL, INSTALL_PROGRAM, INSTALL_DATA): Use
autoconf @-substitutions.