Squelch warning with autoconf > 2.68

Per the discussion at https://autotools.io/forwardporting/autoconf.html
the If-Else family of macros has a new safety switch to ensure that the
source being compiled is setting the expected defines up to that point.

Small change to antlr.m4 ACT_LINK_IFELSE macro.
Merge pull request #6 from terradatum/master

Squelch warning with autoconf > 2.68