checksrc.bat: Better detection of Perl installation

projects/checksrc.bat

@@ -6,7 +6,7 @@ rem *                             / __| | | | |_) | |
 rem *                            | (__| |_| |  _ <| |___
 rem *                             \___|\___/|_| \_\_____|
 rem *
-rem * Copyright (C) 2014, Steve Holme, <steve_holme@hotmail.com>.
+rem * Copyright (C) 2014 - 2015, Steve Holme, <steve_holme@hotmail.com>.
 rem *
 rem * This software is licensed as described in the file COPYING, which
 rem * you should have received as part of this distribution. The terms
@@ -33,8 +33,11 @@ rem ***************************************************************************
 
 :prerequisites
   rem Check we have Perl installed
-  if not exist "C:\Perl" (
-    if not exist "C:\Perl64" goto noperl
+  echo %PATH% | findstr /I /C:"\Perl" 1>nul
+  if errorlevel 1 (
+    if not exist "%SystemDrive%\Perl" (
+      if not exist "%SystemDrive%\Perl64" goto noperl
+    )
   )
 
 :configure