The Compaq compiler (on VMS) includes 'String.c' in source files that
use the stl string while looking for template definitions. This was the
true cause of double-inclusion of the 'kwsysPrivate.h' header. We work
around the problem by conditionally compiling the entire source file on
a condition only true when really building the source.
The kwsysPrivate header double-inclusion check hits a false positive on
VMS for an undetermined reason. This simplifies the workaround and
documents it.