Externals: Update fmt to 6.1.2

Externals/fmt/support/appveyor-build.py

@@ -23,14 +23,17 @@ else:
     # Add MSBuild 14.0 to PATH as described in
     # http://help.appveyor.com/discussions/problems/2229-v140-not-found-on-vs2105rc.
     os.environ['PATH'] = r'C:\Program Files (x86)\MSBuild\15.0\Bin;' + path
-    if image == 'Visual Studio 2013':
-        generator = 'Visual Studio 12 2013'
-    elif image == 'Visual Studio 2015':
-        generator = 'Visual Studio 14 2015'
-    elif image == 'Visual Studio 2017':
-        generator = 'Visual Studio 15 2017'
-    if platform == 'x64':
-        generator += ' Win64'
+    if image == 'Visual Studio 2019':
+        generator = 'Visual Studio 16 2019'
+        if platform == 'x64':
+            cmake_command.extend(['-A', 'x64'])
+    else:
+        if image == 'Visual Studio 2015':
+            generator = 'Visual Studio 14 2015'
+        elif image == 'Visual Studio 2017':
+            generator = 'Visual Studio 15 2017'
+        if platform == 'x64':
+            generator += ' Win64'
     cmake_command.append('-G' + generator)
     build_command = ['cmake', '--build', '.', '--config', config, '--', '/m:4']
     test_command = ['ctest', '-C', config]