projects: Improve Windows perl detection in batch scripts

- Determine if perl is in the user's PATH by running perl.exe.

Prior to this change detection was done by checking the PATH for perl/
but that did not work in all cases (eg git install includes perl but
not in perl/ path).

Bug: https://github.com/curl/curl/pull/2865
Reported-by: Daniel Jeliński

projects/checksrc.bat

@@ -73,7 +73,7 @@ rem ***************************************************************************
 
 :prerequisites
   rem Check we have Perl in our path
-  echo %PATH% | findstr /I /C:"\Perl" 1>nul
+  perl --version <NUL 1>NUL 2>&1
   if errorlevel 1 (
     rem It isn't so check we have it installed and set the path if it is
     if exist "%SystemDrive%\Perl" (