Avoid name shadowing to fix build under powerpc64.

Error was:

cc1: warnings being treated as errors
.../usr.bin/m4/misc.c: In function 'm4errx':
.../usr.bin/m4/misc.c:268: warning: declaration of 'eval' shadows a global declaration
.../usr.bin/m4/extern.h:40: warning: shadowed declaration is here
Temporary revert r269661, it looks like the patch isn't complete.