Fallback to Pure Python if Compilation fails

Originally this code attempted to determine if compiling the C ext
would succeed, and if it thought it should, it would then require that
the C extension succeed in order to install. This fails in cases where
the detection code passes, but compiling ultimately fails (one instance
this might happen is if the Python headers are not installed).

Instead of "asking permission", this code will now just attempt to
compile the module, and will fall back to pure Python if that fails,
unless the person has explicitly asked for the C module, in which case
it will still just fail.

setup.py

@@ -204,63 +204,16 @@ def build_extensions(self):
         self.check_extensions_list(self.extensions)
         for ext in self.extensions:
             with_ext = self.distribution.ext_status(ext)
-            if with_ext is None:
-                with_ext = self.check_extension_availability(ext)
-            if not with_ext:
+            if with_ext is not None and not with_ext:
                 continue
             if with_cython:
                 ext.sources = self.cython_sources(ext.sources, ext)
-            self.build_extension(ext)
-
-    def check_extension_availability(self, ext):
-        cache = os.path.join(self.build_temp, 'check_%s.out' % ext.feature_name)
-        if not self.force and os.path.isfile(cache):
-            data = open(cache).read().strip()
-            if data == '1':
-                return True
-            elif data == '0':
-                return False
-        mkpath(self.build_temp)
-        src = os.path.join(self.build_temp, 'check_%s.c' % ext.feature_name)
-        open(src, 'w').write(ext.feature_check)
-        log.info("checking if %s is compilable" % ext.feature_name)
-        try:
-            [obj] = self.compiler.compile([src],
-                    macros=ext.define_macros+[(undef,) for undef in ext.undef_macros],
-                    include_dirs=ext.include_dirs,
-                    extra_postargs=(ext.extra_compile_args or []),
-                    depends=ext.depends)
-        except CompileError:
-            log.warn("")
-            log.warn("%s is not found or a compiler error: forcing --%s"
-                     % (ext.feature_name, ext.neg_option_name))
-            log.warn("(if %s is installed correctly, you may need to"
-                    % ext.feature_name)
-            log.warn(" specify the option --include-dirs or uncomment and")
-            log.warn(" modify the parameter include_dirs in setup.cfg)")
-            open(cache, 'w').write('0\n')
-            return False
-        prog = 'check_%s' % ext.feature_name
-        log.info("checking if %s is linkable" % ext.feature_name)
-        try:
-            self.compiler.link_executable([obj], prog,
-                    output_dir=self.build_temp,
-                    libraries=ext.libraries,
-                    library_dirs=ext.library_dirs,
-                    runtime_library_dirs=ext.runtime_library_dirs,
-                    extra_postargs=(ext.extra_link_args or []))
-        except LinkError:
-            log.warn("")
-            log.warn("%s is not found or a linker error: forcing --%s"
-                     % (ext.feature_name, ext.neg_option_name))
-            log.warn("(if %s is installed correctly, you may need to"
-                    % ext.feature_name)
-            log.warn(" specify the option --library-dirs or uncomment and")
-            log.warn(" modify the parameter library_dirs in setup.cfg)")
-            open(cache, 'w').write('0\n')
-            return False
-        open(cache, 'w').write('1\n')
-        return True
+            try:
+                self.build_extension(ext)
+            except (CompileError, LinkError):
+                if with_ext is not None:
+                    raise
+                log.warn("Error compiling module, falling back to pure Python")
 
 
 class bdist_rpm(_bdist_rpm):
@@ -342,4 +295,3 @@ def run(self):
         distclass=Distribution,
         cmdclass=cmdclass,
     )
-