Updating to capnproto v0.8.0

- Includes some test stabilization
- Fixes manylinux2010 build issues (linker flag order due to old gcc)
- More rigorous python setup.py clean
- Requires capnproto v0.8.0 or greater
- Including system libcapnp include path for import (e.g. import
  stream_capnp)
- Bundle libcapnp .capnp files when not using system libcapnp
- Removing more distutils usage. Now using pkg-config to determine the
  system version of libcapnp (mainly for Linux, but should work on macOS
  with brew)
- Removed dead code

Resolves issues #215 #216 #217
Lots of fixes for Issue #218 (all sorts of retry methods needed for
GitHub Actions)

buildutils/build.py

@@ -22,7 +22,9 @@ def build_libcapnp(bundle_dir, build_dir):
     os.mkdir(tmp_dir)
 
     cxxflags = os.environ.get('CXXFLAGS', None)
+    ldflags = os.environ.get('LDFLAGS', None)
     os.environ['CXXFLAGS'] = (cxxflags or '') + ' -O2 -DNDEBUG'
+    os.environ['LDFLAGS'] = (ldflags or '')
 
     # Enable ninja for compilation if available
     build_type = []
@@ -77,5 +79,9 @@ def build_libcapnp(bundle_dir, build_dir):
         del os.environ['CXXFLAGS']
     else:
         os.environ['CXXFLAGS'] = cxxflags
+    if ldflags is None:
+        del os.environ['LDFLAGS']
+    else:
+        os.environ['LDFLAGS'] = ldflags
     if returncode != 0:
         raise RuntimeError('capnproto compilation failed: {}'.format(returncode))