This fixes issue #679

This fixes issue #678

Simplifications identified with the help of Coverity

See merge request count0/graph-tool!38

Detect expat using pkg-config

See merge request count0/graph-tool!37

* `AX_LIB_EXPAT` has a big practical issue: It tries to detect expat
  by injecting various paths, among which is /usr/lib. This is extremely
  problematic, as /usr/lib usually contains 32-bit libraries on 64-bit
  systems and thereby kills linkers such as LLD that reject 32-bit
  binaries when linking 64-bit binaries. `pkg-config` does not inject
  /usr/lib, and can also be overridden at configure time.

Closes: https://bugs.gentoo.org/737974

Increase Boost Python function arity only when necessary

See merge request count0/graph-tool!36

Fix dot graph reader

Closes #450

See merge request count0/graph-tool!35

Adjust use of libgvc to fix two graphviz_draw issues

Closes #597

See merge request count0/graph-tool!34

The correct type for the dynamic property map key is actually
graph_property_tag, not multigraph_t*. It looks like graph property maps
only store a single value...

One issue was reported to the mailing list on 2020-08-17 by Rolf
Sander; the testcase is:

import graph_tool.all as gt
g = gt.Graph()
v0 = g.add_vertex()
v1 = g.add_vertex()
e01 = g.add_edge(v0,v1)
e11 = g.add_edge(v1,v1) # loop
gt.graphviz_draw(g)

The other is issue #597 "Graphviz_draw core dumping when using HTML labels"

Basically these changes add extra type information for interacting
with libgvc.