cmake: Add CMakeGraphVizOptions.cmake to improve the generated layout.

Adjust the CMake configuration to generate grapvhiz files with an
improved layout.

Documentation: https://cmake.org/cmake/help/latest/module/CMakeGraphVizOptions.html

To generate a dependency graph from the build directory (example):
cmake . --graphviz=wireshark.dot
fdp wireshark.dot -Tpdf -o wireshark.pdf

Change-Id: Icf238668004224b9d373d8080e549b9b583f676c
Reviewed-on: https://code.wireshark.org/review/30564
Reviewed-by: Peter Wu <peter@lekensteyn.nl>
Reviewed-by: Michael Mann <mmann78@netscape.net>

CMakeGraphVizOptions.cmake

@@ -0,0 +1,11 @@
+# CMake configuration to control the look and contents of generated graphviz files.
+#
+# Documentation: https://cmake.org/cmake/help/latest/module/CMakeGraphVizOptions.html
+#
+# To generate a dependency graph from the build directory:
+#
+#   cmake . --graphviz=wireshark.dot
+#   fdp wireshark.dot -Tpdf -o wireshark.pdf
+
+set(GRAPHVIZ_GRAPH_NAME "Wireshark dependency graph")
+set(GRAPHVIZ_GRAPH_HEADER \tsize="5!" \n\tgraph[splines="true",forcelabels="true",overlap="false"] \n)