No longer rely on pkg-config for libusb

Use AC_CHECK_LIB macro to search for libusb rather than rely on
pkg-config.  Preserve environment variables LIBUSB_CFLAGS and
LIBUSB_LIBS as user precious variables.  Two features,
--with-xml-support and --enable-usrp still rely on pkg-config so only
invoke pkg-config when either or both of these features are selected.
Only the Winradio g313 model uses libdl so test for it only when
Winradio backend is enabled.