doxygen: change the behaviour for caller graphics

This commit is based on a suggestion from Bill Bennet.
With this change doxygen is also creating a call and caller dependency
graphic for global functions. This is helpful for understanding the mode
of action inside the libcoap.

Suggested-by: Bill Bennet <william.bennett@netcommwireless.com>

configure.ac

@@ -162,6 +162,7 @@ if test "x$build_documentation" = "xyes"; then
 				AC_MSG_WARN([==> dot not found - continuing without DOT support])
 				AC_MSG_WARN([==> The libcoap html documentation will be build without DOT graphics!])
 				HAVE_DOT="NO"
+				USE_CALL_GRAPH="NO"
 			else
 				AC_MSG_CHECKING([for compatible dot version])
 				dot_version=`$DOT -V 2>&1|$GREP -oP '(?<=version\W)@<:@0-9.@:>@*(?=\W(.*))'`
@@ -176,8 +177,12 @@ if test "x$build_documentation" = "xyes"; then
 				fi
 				# we found dot and the version is valid
 				HAVE_DOT="YES"
-				# exporting the test to doc/Doxygen(.in)
+				# let doxygen create caller graphics
+				# see http://www.stack.nl/~dimitri/doxygen/manual/config.html#cfg_call_graph
+				USE_CALL_GRAPH="YES"
+				# exporting the tests to doc/Doxygen(.in)
 				AC_SUBST(HAVE_DOT)
+				AC_SUBST(USE_CALL_GRAPH)
 			fi
 		fi
 	fi

doc/Doxyfile.in

@@ -2204,7 +2204,7 @@ 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             = @USE_CALL_GRAPH@
 
 # If the CALLER_GRAPH tag is set to YES then doxygen will generate a caller
 # dependency graph for every global function or class method.
@@ -2215,7 +2215,7 @@ 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           = @USE_CALL_GRAPH@
 
 # If the GRAPHICAL_HIERARCHY tag is set to YES then doxygen will graphical
 # hierarchy of all classes instead of a textual one.