autotools: Workaround a Ubuntu issue when installing in /usr/local/lib

autotools: Workaround a Ubuntu issue when installing in /usr/local/lib

Ubuntu's linker apparently has a bug when installing in /usr/local/lib,
as it is setup in such a way that makes libtool recognize it correctly
as a system path so doesn't add RPATH to our executable, but ld can't
find the library without an explicit ldconfig call (unlike when
installing in /usr/lib).

So, workaround this by explicitly calling ldconfig when installing in
/usr/local/lib -- and when our library is actually not found, to try
and avoid doing something unnecessary on working systems, like e.g.
Debian.