No longer hide aclocal 'underquoted definition' warnings.

http://sources.redhat.com/automake/automake.html#Extending-aclocal documents
that starting with Automake 1.8, aclocal will warn about all underquoted calls
to AC_DEFUN due to the fact that in a single aclocal run it might include more
than once all .m4 files which it finds available, this includes .m4 files from
other software packages.

If the first argument to AC_DEFUN is underquoted and the same macro is included
more than once, successive inclusions after the first one will expand the macro
instead of assuming it is the same as the first one included.