cffi: set LC_ALL=C when invoking preprocessor

This will hopefully normalize line markers to ASCII and fix #95.

I haven't tested this. But setting LC_ALL=C is good for
determinism anyway.

NEWS.rst

@@ -85,6 +85,8 @@ Bug Fixes
   calls ``distutils.sysconfig.customize_compiler()`` so compiler
   customizations (such as honoring the ``CC`` environment variable)
   are performed. Patch by @Arfrever. (#103)
+* The ``make_cffi.py`` script now sets ``LC_ALL=C`` when invoking
+  the preprocessor in an attempt to normalize output to ASCII. (#95)
 
 Changes
 -------

make_cffi.py

@@ -144,6 +144,11 @@ def preprocess(path):
 
     try:
         env = dict(os.environ)
+        # cffi attempts to decode source as ascii. And the preprocessor
+        # may insert non-ascii for some annotations. So try to force
+        # ascii output via LC_ALL.
+        env["LC_ALL"] = "C"
+
         if getattr(compiler, "_paths", None):
             env["PATH"] = compiler._paths
         process = subprocess.Popen(