Fix Berkeley DB version detection:
In #378105 I reported that 'apu-config --libs' prints a list of
dependent libraries, where it should print nothing at all, since you
should not explicitly link to dependent libraries in Debian. Tollef
said he'd fix it.
However, BDB version detection relies on this behavior, as it runs
'apu-config --libs', pulls out the -ldb* word, and compiles a test
program with this. By adding 'apu-config --link-ld', which will be
-laprutil-1, that will pull in libdb4.x regardless of whether
'apu-config --libs' prints it explicitly.