Externals: Add libfmt 5.3.0 to externals

Allows us to migrate off of printf specifiers and have more type-safe
formatting facilities. It also allows for custom type support as well.
fmt is also on track to have part of it standardized within C++2a, so
this will also lessen the transitional work necessary later on by
allowing new code to use it.

This simply adds the library but doesn't do anything with it yet.

Externals/fmt/support/appveyor-build.py

@@ -0,0 +1,40 @@
+#!/usr/bin/env python
+# Build the project on AppVeyor.
+
+import os
+from subprocess import check_call
+
+build = os.environ['BUILD']
+config = os.environ['CONFIGURATION']
+platform = os.environ['PLATFORM']
+path = os.environ['PATH']
+image = os.environ['APPVEYOR_BUILD_WORKER_IMAGE']
+jobid = os.environ['APPVEYOR_JOB_ID']
+cmake_command = ['cmake', '-DFMT_PEDANTIC=ON', '-DCMAKE_BUILD_TYPE=' + config, '..']
+if build == 'mingw':
+    cmake_command.append('-GMinGW Makefiles')
+    build_command = ['mingw32-make', '-j4']
+    test_command = ['mingw32-make', 'test']
+    # Remove the path to Git bin directory from $PATH because it breaks
+    # MinGW config.
+    path = path.replace(r'C:\Program Files (x86)\Git\bin', '')
+    os.environ['PATH'] = r'C:\MinGW\bin;' + path
+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'
+    cmake_command.append('-G' + generator)
+    build_command = ['cmake', '--build', '.', '--config', config, '--', '/m:4']
+    test_command = ['ctest', '-C', config]
+
+check_call(cmake_command)
+check_call(build_command)
+check_call(test_command)