tests: increase coverage for depcomp tests

tests: increase coverage for depcomp tests

On platforms requiring that no undefined symbols exist in order
to build shared libraries (e.g. Windows DLLs), you have to
explicitly declare that the libtool library you are building
does not actually have any undefined symbols, for libtool to
even try to build it as a shared library.  Without that
explicit declaration, libtool falls back to a static library
only, regardless of any --enable-shared flags etc.

tests/depcomp.sh (Makefile.am): Add -no-undefined if a libtool
library (.la) is built.
(src/Makefile.am): Likewise.