Don't set up environment for ninja if the compiler is already available

ninja.bat

Index: ninja.bat
===================================================================
--- ninja.bat	(revision 133516)
+++ ninja.bat	(working copy)
@@ -25,18 +25,33 @@
 )
 :done_dash_c
 
+:: Try running the compiler. If it fails, then we assume we need to set up the
+:: environment for the compiler.
+:: TODO(scottmg): We should also try to detect if we have the right version
+:: of the compiler too (i.e. If generation specified 2010, but we're running
+:: from a 2008 command prompt).
+cl 2>nul >nul
+if not errorlevel 1 goto no_set_env
+
 if not exist "%cd_path%\set_environment.bat" (
   echo ninja.bat: set_environment.bat not found in '%cd_path%'. Is -C arg correct?
   goto :EOF
 )
 call "%cd_path%\set_environment.bat"
 
+:: Export only the path changes out of the script.
+endlocal & set PATH=%PATH% & set INCLUDE=%INCLUDE% & set LIBPATH=%LIBPATH% & set LIB=%LIB%
+
+:: To pair with below when we don't skip this block.
+setlocal
+
 :: Add python to the path, many gyp rules assume it's there.
 :: Add ninja directory to the path (to find ninja and ninja-deplist-helper).
-:: Then, export only the path changes out of the script so that next time we
-:: just run ninja directly (otherwise, this script adds about 500-800ms to
+:: Put it at the front so that ninja.exe is found before this wrapper so that
+:: next time we just run it directly (otherwise, this script adds 500-800ms to
 :: ninja invocations).
-endlocal & set PATH=%~dp0python_bin;%~dp0ninja-win;%PATH% & set INCLUDE=%INCLUDE% & set LIBPATH=%LIBPATH% & set LIB=%LIB%
+:no_set_env
+endlocal & set PATH=%~dp0python_bin;%~dp0ninja-win;%PATH%
 
 :: Now run the actual build.
 ninja.exe %*