misc: better parameterization for toolset version

Change-Id: Iedb6600c6c17bf35aa0a4551caf4bb5050c8e801

compat/msvc/winmake.py

@@ -67,7 +67,7 @@ patch_args = ['-flp1', '-i']
 
 # vs help
 win_sdk_default = '10.0.16299.0'
-win_toolset_default = 'v142'
+win_toolset_default = '142'
 
 vs_where_path = os.path.join(
     os.environ['ProgramFiles(x86)'], 'Microsoft Visual Studio', 'Installer', 'vswhere.exe'
@@ -730,6 +730,10 @@ def parse_args():
 
     parsed_args = ap.parse_args()
 
+    if parsed_args.toolset:
+        if parsed_args.toolset[0] != 'v':
+            parsed_args.toolset = 'v' + parsed_args.toolset
+
     return parsed_args