Don't set up MSVC environment *after* locating tools

And warn if no Visual C++ installation is detected, similar to the now
suppressed warning issued by the batch file.

driver/tool.cpp

@@ -209,7 +209,7 @@ int executeAndWait(const char *commandLine) {
   return exitCode;
 }
 
-bool setupMsvcEnvironment() {
+bool setupMsvcEnvironmentImpl() {
   if (getenv("VSINSTALLDIR"))
     return true;
 
@@ -311,6 +311,13 @@ bool setupMsvcEnvironment() {
   return haveVsInstallDir;
 }
 
+bool setupMsvcEnvironment() {
+  const bool success = setupMsvcEnvironmentImpl();
+  if (!success)
+    warning(Loc(), "no Visual C++ installation detected");
+  return success;
+}
+
 } // namespace windows
 
 #endif // _WIN32