Doxygen: add some options for Doxyfile generation

`make doc` now creates collaboration and inheritance graphs by default

Run cmake with -DDOC_GRAPHS=No to disable all graphs and run use
-DDOCS_FULLGRAPHS=Yes to also generate call/callee graphs (large)

CMakeLists.txt

@@ -225,6 +225,16 @@ endif()
 # Build Documentation
 
 if(BUILD_DOCUMENTATION)
+  set(DOC_GRAPHS "YES" CACHE STRING "Create dependency graphs (needs graphviz)")
+  set(DOC_FULLGRAPHS "NO" CACHE STRING "Create call/callee graphs (large)")
+
+  find_program(DOT_PATH dot)
+
+  if (DOT_PATH STREQUAL "DOT_PATH-NOTFOUND")
+    message("Doxygen: graphviz not found - graphs disabled")
+    set(DOC_GRAPHS "NO")
+  endif()
+
   find_package(Doxygen)
   if(DOXYGEN_FOUND)
     configure_file("cmake/Doxyfile.in" "Doxyfile" @ONLY)

cmake/Doxyfile.in

@@ -2049,7 +2049,7 @@ HIDE_UNDOC_RELATIONS   = YES
 # set to NO
 # The default value is: NO.
 
-HAVE_DOT               = NO
+HAVE_DOT               = @DOC_GRAPHS@
 
 # The DOT_NUM_THREADS specifies the number of dot invocations doxygen is allowed
 # to run in parallel. When set to 0 doxygen will base this on the number of
@@ -2165,7 +2165,7 @@ INCLUDED_BY_GRAPH      = YES
 # The default value is: NO.
 # This tag requires that the tag HAVE_DOT is set to YES.
 
-CALL_GRAPH             = NO
+CALL_GRAPH             = @DOC_FULLGRAPHS@
 
 # If the CALLER_GRAPH tag is set to YES then doxygen will generate a caller
 # dependency graph for every global function or class method.
@@ -2176,7 +2176,7 @@ CALL_GRAPH             = NO
 # The default value is: NO.
 # This tag requires that the tag HAVE_DOT is set to YES.
 
-CALLER_GRAPH           = NO
+CALLER_GRAPH           = @DOC_FULLGRAPHS@
 
 # If the GRAPHICAL_HIERARCHY tag is set to YES then doxygen will graphical
 # hierarchy of all classes instead of a textual one.