LP1990018: Fix libdbi build

LP1990018: Fix libdbi build

Resolve issues with finding libdbi and libdbdpgsql on ARM64 by using better
autoconf hygiene:

 * Use PKG_CHECK_MODULES to find and set the dbi library flags.
 * Remove the "backup" code to find a manually installed libdbi.
   (This has not been needed for some time.)
 * Update Makefile.ams to use the dbi LDFLAGS properly.
 * Remove -ldbdpgsql from compile commands.
   (It is loaded dynamically.)

I tested it on a x86_64 VM with Ubuntu 22.04.  Most tests passed.  Only the
remot auth and cover uploader tests failed.  The latter have not passed for
some time, and I believe the former may have been affected by a recent
change unrelated to this commit.

Signed-off-by: Jason Stephenson <jason@sigio.com>