I spent appx. 2 hours browsing Google search results, quickly
rejecting most stuff because it's not really C++ (it's C), or because
it's not for Unix (it's for Windows only), or because it's something
other than a basic set of routines for access to CGI service from C++.


I almost downloaded each of three, but then after much investigation
I finally rejected each and moved onward in the Google search results:

Looks good, fully OO, but too large so I haven't tried it yet:
  http://www.webthing.com/cgiplusplus/

Looks simpler hence better for my purpose, but still large:
  http://hjs.geol.uib.no/CplusPlus/

Looks more "together", but specific for Linux, not FreeBSD Unix:
  ftp://ftp.cgicc.org/


Finally I picked this one and downloaded it:
  http://www.p3ptools.com/?category=cplus
Then I modified it to generate Valid HTML 4.01 Transitional
per this validator service:
  http://validator.w3.org/
and added a routine copy_value to copy the value from get_value so that
returned values don't clobber each other.