scripts/kerneldoc: For Sphinx 3 use c:macro for macros with arguments

The kerneldoc script currently emits Sphinx markup for a macro with
arguments that uses the c:function directive. This is correct for
Sphinx versions earlier than Sphinx 3, where c:macro doesn't allow
documentation of macros with arguments and c:function is not picky
about the syntax of what it is passed. However, in Sphinx 3 the
c:macro directive was enhanced to support macros with arguments,
and c:function was made more picky about what syntax it accepted.

When kerneldoc is told that it needs to produce output for Sphinx
3 or later, make it emit c:function only for functions and c:macro
for macros with arguments. We assume that anything with a return
type is a function and anything without is a macro.

This fixes the Sphinx error:

/home/petmay01/linaro/qemu-from-laptop/qemu/docs/../include/qom/object.h:155:Error in declarator
If declarator-id with parameters (e.g., 'void f(int arg)'):
  Invalid C declaration: Expected identifier in nested name. [error at 25]
    DECLARE_INSTANCE_CHECKER ( InstanceType,  OBJ_NAME,  TYPENAME)
    -------------------------^
If parenthesis in noptr-declarator (e.g., 'void (*f(int arg))(double)'):
  Error in declarator or parameters
  Invalid C declaration: Expecting "(" in parameters. [error at 39]
    DECLARE_INSTANCE_CHECKER ( InstanceType,  OBJ_NAME,  TYPENAME)
    ---------------------------------------^

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Tested-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 20201030174700.7204-2-peter.maydell@linaro.org

scripts/kernel-doc

@@ -839,7 +839,23 @@ sub output_function_rst(%) {
 	output_highlight_rst($args{'purpose'});
 	$start = "\n\n**Syntax**\n\n  ``";
     } else {
-	print ".. c:function:: ";
+        if ((split(/\./, $sphinx_version))[0] >= 3) {
+            # Sphinx 3 and later distinguish macros and functions and
+            # complain if you use c:function with something that's not
+            # syntactically valid as a function declaration.
+            # We assume that anything with a return type is a function
+            # and anything without is a macro.
+            if ($args{'functiontype'} ne "") {
+                print ".. c:function:: ";
+            } else {
+                print ".. c:macro:: ";
+            }
+        } else {
+            # Older Sphinx don't support documenting macros that take
+            # arguments with c:macro, and don't complain about the use
+            # of c:function for this.
+            print ".. c:function:: ";
+        }
     }
     if ($args{'functiontype'} ne "") {
 	$start .= $args{'functiontype'} . " " . $args{'function'} . " (";