configure: use a convenience library for fixed point arithmetic

automake >= 1.14 does not like building binaries from source in other
directories. To avoid a warning, `subdir-objects` option has to be
passed to `AM_INIT_AUTOMAKE`. Unfortunately, this breaks what worked
before. Fixed point tests are using `fixedpoint.c` from
`src/lib`. This seems to confuse dependency tracking. See:

 http://debbugs.gnu.org/cgi/bugreport.cgi?bug=13928
 http://mytestbed.net/issues/1327

As a workaround, we use a convenience library instead.