cffi: Use the distutils preprocessor when available

On Unix like systems, distutils will usually set the preprocessor
attribute to the appropriate command. On MSVC it never sets the
preprocessor, though. In either case, try to use it and only fallback to
using the compiler if needed. Using compiler.compiler[0] breaks use of
ccache when inserted in the CC environment variable.

This is a regression from 0d1329a.

Closes #178.
Closes #179.

docs/news.rst

@@ -89,6 +89,8 @@ Changes
 * Anaconda 3.6 support dropped.
 * Official support for Python 3.11. This did not require meaningful code changes
   and previous release(s) likely worked with 3.11 without any changes.
+* CFFI's build system now respects distutils's ``compiler.preprocessor`` if it
+  is set. (#179)
 
 0.18.0 (released 2022-06-20)
 ============================

make_cffi.py

@@ -40,23 +40,24 @@
 # environment variables like CC.
 distutils.sysconfig.customize_compiler(compiler)
 
-# Distutils doesn't set compiler.preprocessor, so invoke the preprocessor
-# manually.
+# Distutils doesn't always set compiler.preprocessor, so invoke the
+# preprocessor manually when needed.
+args = getattr(compiler, "preprocessor", None)
 if compiler.compiler_type == "unix":
-    # Using .compiler respects the CC environment variable.
-    args = [compiler.compiler[0]]
+    if not args:
+        # Using .compiler respects the CC environment variable.
+        args = [compiler.compiler[0], "-E"]
     args.extend(
         [
-            "-E",
             "-DZSTD_STATIC_LINKING_ONLY",
             "-DZDICT_STATIC_LINKING_ONLY",
         ]
     )
 elif compiler.compiler_type == "msvc":
-    args = [compiler.cc]
+    if not args:
+        args = [compiler.cc, "/EP"]
     args.extend(
         [
-            "/EP",
             "/DZSTD_STATIC_LINKING_ONLY",
             "/DZDICT_STATIC_LINKING_ONLY",
         ]