Stop using VOIDUSED in the headers.

VOIDUSED was provided by metaconfig to work around pre-ANSI compilers that
didn't support all uses of the void type. We've been mandating a proper ANSI
C89 compiler for at least a decade, so we don't need this any more.
Eliminate the only use of VOIDFLAGS, as part of STMT_START in perl.h

STMT_START has used VOIDFLAGS as part of its conditional compilation since it
was added by commit 728e280303ae3d3b (March 1996). The code originally read:

   /* Now which other defined()s do we need here ??? */

Since then it has been amended to avoid entering that definition for GCC
on Solaris. Given that all current Solaris compilers are C89 conformant,
VOIDFLAGS will always be true (actually 15), so the test is redundant.
Even back in 1996, it's possible that VOIDFLAGS was always non-zero on
SunOS, rendering the test obsolete from the start.

Spotted by Brian Fraser, and extracted from a larger patch of his.
Eliminate the use of voidflags from Configure's pointer size determination.

This should permit metaconfig to drop the entire voidflags logic from
Configure and the config.sh related files.