OpenWatcom: Add workaround for lack of error on missing library

The OpenWatcom tools do not fail to link when a library is missing if no
symbols were needed from it.  This can break `try_compile` checks.
Teach `cmGlobalGenerator::Build` to pretend that the build tool returned
non-zero if the output contains the corresponding warning.

Source/cmGlobalGenerator.cxx

@@ -1893,6 +1893,13 @@ int cmGlobalGenerator::Build(const std::string& /*unused*/,
     retVal = 1;
   }
 
+  // The OpenWatcom tools do not return an error code when a link
+  // library is not found!
+  if (this->CMakeInstance->GetState()->UseWatcomWMake() && retVal == 0 &&
+      output.find("W1008: cannot open") != std::string::npos) {
+    retVal = 1;
+  }
+
   return retVal;
 }