build: update AC_PACKAGE_WANT_GDBM() and src/dbtest.c to build

build: update AC_PACKAGE_WANT_GDBM() and src/dbtest.c to build

Modern gdbm-devel packages bundle together gdbm.h and ndbm.h.  The
old m4 macro had detection support for some old gdbm libraries but
not for new ones.

We fix compilation of src/dbtest.c by making the autoconf helper
check for this new arrangement:

If both gdbm.h and ndbm.h are found define set both gdbm_ndbm_=true,
and have_db=true, and define HAVE_GDBM_H. The src/dbtest.c already
had a HAVE_GDBM_H but there was never a respective autoconf settter
for it. We can just re-use this and fix it for new arrangement.

Signed-off-by: Luis R. Rodriguez <mcgrof@kernel.org>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>