Rework mach.bat to support VS2019 and user-supplied environments.

As a bonus, use setlocal to avoid environment pollution.

mach.bat

@@ -1,5 +1,11 @@
 @echo off
 
+setlocal
+
+if EXIST "%VCINSTALLDIR%" (
+  GOTO mach
+)
+
 pushd .
 
 IF EXIST "%ProgramFiles(x86)%" (
@@ -8,37 +14,52 @@ IF EXIST "%ProgramFiles(x86)%" (
   set "ProgramFiles32=%ProgramFiles%"
 )
 
-set VC14VARS=%VS140COMNTOOLS%..\..\VC\vcvarsall.bat
-IF EXIST "%VC14VARS%" (
-  set "VS_VCVARS=%VC14VARS%"
-) ELSE (
+for %%v in (2019 2017) do (
   for %%e in (Enterprise Professional Community BuildTools) do (
-    IF EXIST "%ProgramFiles32%\Microsoft Visual Studio\2017\%%e\VC\Auxiliary\Build\vcvarsall.bat" (
-      set "VS_VCVARS=%ProgramFiles32%\Microsoft Visual Studio\2017\%%e\VC\Auxiliary\Build\vcvarsall.bat"
+    IF EXIST "%ProgramFiles32%\Microsoft Visual Studio\%%v\%%e\VC\Auxiliary\Build\vcvarsall.bat" (
+      set "VS_VCVARS=%ProgramFiles32%\Microsoft Visual Studio\%%v\%%e\VC\Auxiliary\Build\vcvarsall.bat"
+      GOTO vcvars
     )
   )
 )
 
+set VC14VARS=%VS140COMNTOOLS%..\..\VC\vcvarsall.bat
+IF EXIST "%VC14VARS%" (
+  set "VS_VCVARS=%VC14VARS%"
+)
+
+:vcvars
 IF EXIST "%VS_VCVARS%" (
   IF NOT DEFINED Platform (
     IF EXIST "%ProgramFiles(x86)%" (
       call "%VS_VCVARS%" x64
     ) ELSE (
       ECHO 32-bit Windows is currently unsupported.
-      EXIT /B 1
+      GOTO bad_exit
     )
   )
 ) ELSE (
-  ECHO Visual Studio 2015 or 2017 is not installed.
-  ECHO Download and install Visual Studio 2015 or 2017 from https://www.visualstudio.com/
-  EXIT /B 1
+  ECHO Visual Studio 2015, 2017, or 2019 is not installed.
+  ECHO Download and install Visual Studio from https://www.visualstudio.com/
+  GOTO bad_exit
 )
 
 popd
 
+:mach
 where /Q py.exe
 IF %ERRORLEVEL% NEQ 0 (
   python mach %*
 ) ELSE (
   py -2 mach %*
 )
+
+GOTO exit
+
+:bad_exit
+endlocal
+EXIT /B 1
+
+:exit
+endlocal
+exit /B