Prefer non-empty prefixes when matching lib names (#11468)

In cmComputeLinkInformation we match library names with a regular
expression, possibly extracting the 'lib' prefix.  The regex component
to match the prefix always allows an empty prefix to be matched, as in
"(lib|)".  Avoid every adding an empty prefix option earlier in the
regex, as in "(|lib|)", because it will be preferred and 'lib' will
never match.

Source/cmComputeLinkInformation.cxx

@@ -923,7 +923,7 @@ void cmComputeLinkInformation::ComputeItemParserInfo()
 //----------------------------------------------------------------------------
 void cmComputeLinkInformation::AddLinkPrefix(const char* p)
 {
-  if(p)
+  if(p && *p)
     {
     this->LinkPrefixes.insert(p);
     }