Fallback to building pure Python variant if extension cannot be compiled.

setup.py

@@ -1,10 +1,36 @@
+from __future__ import print_function
+
 import os
+import sys
 
 from distutils.core import setup
 from distutils.core import Extension
+from distutils.command.build_ext import build_ext
+from distutils.errors import (CCompilerError, DistutilsExecError,
+                DistutilsPlatformError)
 
-with_extensions = os.environ.get('WRAPT_EXTENSIONS', 'true')
-with_extensions = (with_extensions.lower() != 'false')
+if sys.platform == 'win32':
+    build_ext_errors = (CCompilerError, DistutilsExecError,
+            DistutilsPlatformError, IOError)
+else:
+    build_ext_errors = (CCompilerError, DistutilsExecError,
+            DistutilsPlatformError)
+
+class BuildExtFailed(Exception):
+    pass
+
+class optional_build_ext(build_ext):
+    def run(self):
+        try:
+            build_ext.run(self)
+        except DistutilsPlatformError:
+            raise BuildExtFailed()
+
+    def build_extension(self, ext):
+        try:
+            build_ext.build_extension(self, ext)
+        except build_ext_errors:
+            raise BuildExtFailed()
 
 setup_kwargs = dict(
       name = 'wrapt',
@@ -18,11 +44,56 @@ setup_kwargs = dict(
       package_dir={'wrapt': 'src'},
      )
 
-setup_extension_kwargs = dict(
-    ext_modules = [Extension("wrapt._wrappers", ["src/_wrappers.c"])],
-)
+def run_setup(with_extensions):
+    setup_kwargs_tmp = dict(setup_kwargs)
+
+    if with_extensions:
+        setup_kwargs_tmp['ext_modules'] = [
+                Extension("wrapt._wrappers", ["src/_wrappers.c"])]
+        setup_kwargs_tmp['cmdclass'] = dict(build_ext=optional_build_ext)
+
+    setup(**setup_kwargs_tmp)
+
+with_extensions = os.environ.get('WRAPT_EXTENSIONS', None)
 
 if with_extensions:
-    setup_kwargs.update(setup_extension_kwargs)
+    if with_extensions.lower() == 'true':
+        with_extensions = True
+    elif with_extensions.lower() == 'false':
+        with_extensions = False
+    else:
+        with_extensions = None
 
-setup(**setup_kwargs)
+if hasattr(sys, 'pypy_version_info'):
+    with_extensions = False
+
+WARNING = """
+WARNING: The C extension component for wrapt could not be compiled.
+"""
+
+if with_extensions is not None:
+    run_setup(with_extensions=with_extensions)
+
+else:
+    try:
+        run_setup(with_extensions=True)
+
+    except BuildExtFailed:
+
+        print(75 * '*')
+
+        print(WARNING)
+        print("INFO: Trying to build without extensions.")
+
+        print()
+        print(75 * '*')
+
+        run_setup(with_extensions=False)
+
+        print(75 * '*')
+
+        print(WARNING)
+        print("INFO: Only pure Python version of package installed.")
+
+        print()
+        print(75 * '*')