Reset xxx_convert to empty string if none of 3 utilities is found

Reset xxx_convert to empty string if none of 3 utilities is found

This will cause Configure to enter the WHOA block and default to using
sprintf.

Change Gconvert checkit() prototype for [perl #134371].

In the checkit() routine inside Configure, clang++ was taking the
if (strcmp(expect, got)) branch even though the 'expect' and 'got' strings
were identical.  A first step in debugging this was to realize that
the checkit() function never returned a value, so relabel it as void.
With clang version 7.0.1-8 (tags/RELEASE_701/final) (Debian),
this change seems to work around the strcmp issue.

Further Configure gconvert probe cleanups for C++.  As part of debugging
[perl #134375] and [perl #134371], try to eliminate warnings thrown by
clang++.