- fix usage message on --with-mechglue

- fix usage message on --with-mechglue
- list full path to mechglue libgssapi.a on link line rather than using
  -L${mechglue_path} because we don't want to pick up some other libgssapi.so
  which some linkers will give precendence to
- pass in /bin/true as then case of AC_CHECK_LIB because the default
  appends the library to the linker path which isn't what we want