Building Samba 4.4.6 or 4.5.0 on Solaris 10 using the Sun C compiler I get 79029 warnings:

warning:  attribute "format" is unknown, ignored

This happens with both the Sun C 5.11 and Sun C 5.13 compilers on both Sparc and i386 platforms. This looks to be due to changes in the code such as:

-#if (__GNUC__ >= 3) && (__GNUC_MINOR__ >= 1 )
+#ifdef HAVE___ATTRIBUTE__

and the fact that the test for __attribute__ suceeds. The test is:

            void test_attribute(void) __attribute__ (());

            void test_attribute(void)
            {
                return;
            }

            int main(void) {
                return 0;
            }

And this does compile without any warnings. So it looks like the Sun C compilers do have attribute, just not attribute format.