Statically link the VC runtime

Before switching to distutils, we were statically linking the
VC runtime.

distutils, on the other hand, is enforcing dynamic linking. The
reason is that some extensions were hitting some runtime
limitations when statically linking it:
https://bugs.python.org/issue38597

This admittedly hacky patch will override the hardcoded distutils
/MD flag. It should work with all the supported Python versions.
For future versions, we're probably going to request the Python
community a better way of statically linking the runtime when
aware of the limitations.

Author: petrutlucian94

PyMI/README.rst

@@ -122,6 +122,12 @@ version, use the following:
 
     python setup.py bdist_wheel
 
+Make sure to use the Visual Studio toolset that matches the Python version
+that you're targetting: https://wiki.python.org/moin/WindowsCompilers.
+
+By default, we're statically linking the VC runtime. To enable dynamic
+linking, set ``$env:PYMI_VCRUNTIME_DYNAMIC_LINKING="y"``.
+
 Debug builds
 ^^^^^^^^^^^^
 

setup.py

@@ -1,3 +1,5 @@
+from distutils import _msvccompiler
+from distutils import ccompiler
 import os
 import setuptools
 
@@ -6,6 +8,38 @@
 except ImportError:
     pass
 
+
+# MSVCP140.dll isn't included by Python along with the C runtime,
+# for which reason we're going to statically link it for now.
+# This can be disabled using the PYMI_VCRUNTIME_DYNAMIC_LINKING
+# flag.
+# Unfortunately distutils harcodes the "/MD" flag.
+class Compiler(_msvccompiler.MSVCCompiler):
+    def initialize(self, *args, **kwargs):
+        super(Compiler, self).initialize(*args, **kwargs)
+
+        dynamically_link_runtime = os.environ.get(
+          "PYMI_VCRUNTIME_DYNAMIC_LINKING", "").lower() in (
+          "yes", "y", "1", "true", "t")
+        if not dynamically_link_runtime:
+            self._statically_link_runtime()
+
+    def _statically_link_runtime(self):
+        if '/MD' in self.compile_options:
+          self.compile_options.remove('/MD')
+          self.compile_options.append('/MT')
+        if '/MDd' in self.compile_options_debug:
+          self.compile_options_debug.remove('/MDd')
+          self.compile_options_debug.append('/MTd')
+
+
+def new_compiler(plat=None, compiler=None, verbose=0, dry_run=0, force=0):
+    return Compiler(None, dry_run, force)
+
+
+ccompiler.new_compiler = new_compiler
+
+
 # Setuptools requires relative paths
 mi_dir = 'MI'
 pymi_dir = 'PyMI'