Fix bug in configure.ac for enable-poll -- it sets the wrong variable on false.
Fix bug in configure.ac for enable-poll -- it sets the wrong variable on false.

thewhimer/gambit,acenode/gambit,acenode/gambit,acenode/gambit,BeImprovised/gambit,BeImprovised/gambit,thewhimer/gambit,jasperla/gambit,thewhimer/gambit,acenode/gambit,jasperla/gambit,thewhimer/gambit,jasperla/gambit,thewhimer/gambit,BeImprovised/gambit,jasperla/gambit,thewhimer/gambit,BeImprovised/gambit