Silence flex warnings about DOS file paths in MSVC builds

src/tools/msvc/pgflex.bat

 @echo off
 REM src/tools/msvc/pgflex.bat
 
+REM silence flex bleatings about file path style
+SET CYGWIN=nodosfilewarning
+
 IF NOT EXIST src\tools\msvc\buildenv.pl goto nobuildenv
 perl -e "require 'src/tools/msvc/buildenv.pl'; while(($k,$v) = each %ENV) { print qq[\@SET $k=$v\n]; }" > bldenv.bat
 CALL bldenv.bat