argp: make HAVE_DECL usage consistent

argp: make HAVE_DECL usage consistent

Autoconf guarantees that AC_CHECK_DECLS always defines the
corresponding HAVE_DECL_*.  But our hand-coded use of AC_CHECK_DECL
did not define the macro to 0, leading to a false positive complaint:
https://lists.gnu.org/archive/html/coreutils/2012-06/msg00037.html
By sticking to conventions, we avoid the false positive.

* lib/argp-parse.c (__argp_parse): Check contents of HAVE_DECL
macros, not whether they are defined.
* m4/argp.m4 (gl_ARGP): Always define HAVE_DECL_* macros, per
convention with other declaration checks.
Reported by Karel Zak, with suggestions from Paul Eggert.