Add additional include path

[adarshp]

Adding an additional include path to find graphviz/cgraph.h. For example, running

pkg-config --cflags-only-I libcgraph

with MacPorts returns

-I/opt/local/include/graphviz

(and similarly for Homebrew, except for usr instead of opt)

[coveralls]

Coverage remained the same at 82.001% when pulling fa41efb on adarshp:patch-1 into 37208d1 on pygraphviz:master.

[coveralls]

Coverage remained the same at 82.001% when pulling fa41efb on adarshp:patch-1 into 37208d1 on pygraphviz:master.

[dschult]

This seems reasonable to me -- though I wonder how many of these we might eventually get.
Is there a way to make pkg-config return what we want? We could also merge this and then add an issue to explore other ways of finding include paths.

[adarshp]

This solution does use pkg-config though - it doesn't add any hard-coded paths.

The command pkg-config --cflags-only-I libcgraph returns the following:

• -I/opt/local/include/graphviz on macOS with MacPorts
• -I/usr/include/graphviz on CentOS and Ubuntu

The line below:

pygraphviz/pygraphviz/graphviz_wrap.c

Line 2676 in f5033cd

#include "graphviz/cgraph.h"

Seems to work on Linux, but I think (and this is just a guess) that is because it is using /usr/local/include as a default path to search for include declarations (in this case, it would find <graphviz/cgraph.h> whether or not pkg-config was used). To really utilize the info provided by pkg-config and not implicitly rely on the /usr/local prefix, I would suggest changing

#include <graphviz/cgraph.h>

to

#include <cgraph.h>