configure.ac: Rename AM_LDFLAGS to LTLDFLAGS and actually use them

The new variable name tries to clarify that libtool is being used.
Linker flags must thus always be specified with -Wl.

Factor out the libtool flag -no-undefined from host specific cases.
The flag is required to build a Windows DLL, but is correct also for
the other supported systems.

Also, start actually using LTLDFLAGS in libusb/Makefile.am, so that
libtool will see the options set by configure.