Add automatic build of libcapnp if it's not detected

setup.py

@@ -1,4 +1,6 @@
 #!/usr/bin/env python
+from __future__ import print_function
+
 try:
     from Cython.Build import cythonize
     import Cython
@@ -15,12 +17,18 @@ if setuptools_version < '0.8':
 
 from distutils.core import setup
 import os
+from buildutils import test_build, fetch_libcapnp, build_libcapnp, info
+from distutils.errors import CompileError
+from distutils.extension import Extension
+from Cython.Distutils import build_ext as build_ext_c
+_this_dir = os.path.dirname(__file__)
 
 MAJOR = 0
 MINOR = 4
 MICRO = 6
 VERSION = '%d.%d.%d' % (MAJOR, MINOR, MICRO)
 
+# Write version info
 def write_version_py(filename=None):
     cnt = """\
 version = '%s'
@@ -43,12 +51,14 @@ from .lib.capnp import _CAPNP_VERSION as LIBCAPNP_VERSION
 
 write_version_py()
 
+# Try to convert README using pandoc
 try:
     import pypandoc
     long_description = pypandoc.convert('README.md', 'rst')
 except (IOError, ImportError):
     long_description = ''
 
+# Clean command, invoked with `python setup.py clean`
 from distutils.command.clean import clean as _clean
 class clean(_clean):
     def run(self):
@@ -60,6 +70,31 @@ class clean(_clean):
             except OSError:
                 pass
 
+
+class build_libcapnp_ext(build_ext_c):
+
+    def build_extension(self, ext):
+        build_ext_c.build_extension(self, ext)
+
+    def run(self):
+        try:
+            test_build()
+        except CompileError:
+            info("*WARNING* no libcapnp detected. Will download and build it from source now. If you have C++ Cap'n Proto installed, it may be out of date or is not being detected. Downloading and building libcapnp may take a while.")
+            bundle_dir = os.path.join(_this_dir, "bundled")
+            if not os.path.exists(bundle_dir):
+                os.mkdir(bundle_dir)
+            build_dir = os.path.join(_this_dir, "build")
+            if not os.path.exists(build_dir):
+                os.mkdir(build_dir)
+            fetch_libcapnp(bundle_dir)
+
+            build_libcapnp(bundle_dir, build_dir)
+
+            self.include_dirs += [os.path.join(build_dir, 'include')]
+            self.library_dirs += [os.path.join(build_dir, 'lib')]
+
+        return build_ext_c.run(self)
 setup(
     name="pycapnp",
     packages=["capnp"],
@@ -67,7 +102,8 @@ setup(
     package_data={'capnp': ['*.pxd', '*.h', '*.capnp', 'helpers/*.pxd', 'helpers/*.h', 'includes/*.pxd', 'lib/*.pxd', 'lib/*.py', 'lib/*.pyx']},
     ext_modules=cythonize('capnp/lib/*.pyx'),
     cmdclass = {
-        'clean': clean
+        'clean': clean,
+        'build_ext': build_libcapnp_ext
     },
     install_requires=[
         'cython >= 0.21',