build: clean up AC_PACKAGE_WANT_GDBM

build: clean up AC_PACKAGE_WANT_GDBM

The AC_PACKAGE_WANT_GDBM macro is not easily read. It's not doing
anything particularly complex other than working through a set of
alternatives for headers and libraries.

This patch cleans it up to be more readable.  We also only attempt
to check in libgdbm_compat if the checks in libgdbm fail.

Signed-off-by: Jeff Mahoney <jeffm@suse.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>