Bug 654196 - "make check" fails when built with --enable-dbi

but without sqlite DBD installed.
Refine checks to use the values for dlopen and dlopen_lib already
discovered by libtool.
Improve the error message to offer more solutions.

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/trunk@23190 57a11ea4-9604-0410-9ed3-97b8803252fd