configure: Fix build with automake < 1.11

Commit 86d30dea3c13d29ef8d39bc18db63a0441051975 broke building with older
automake versions with this error:

Makefile:769: *** Recursive variable am__v_YACC_ references itself (eventually).  Stop.

This patch fixes it. Fix stolen from xorg-macros.

Signed-off-by: Lauri Kasanen <cand@gmx.com>
1 file changed