kbuild: document cc-fullversion

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>

Documentation/kbuild/makefiles.txt

@@ -501,6 +501,20 @@ more details, with real examples.
 	The third parameter may be a text as in this example, but it may also
 	be an expanded variable or a macro.
 
+    cc-fullversion
+	cc-fullversion is useful when the exact version of gcc is needed.
+	One typical use-case is when a specific GCC version is broken.
+	cc-fullversion points out a more specific version than cc-version does.
+
+	Example:
+		#arch/powerpc/Makefile
+		$(Q)if test "$(call cc-fullversion)" = "040200" ; then \
+			echo -n '*** GCC-4.2.0 cannot compile the 64-bit powerpc ' ; \
+			false ; \
+		fi
+
+	In this example for a specific GCC version the build will error out explaining
+	to the user why it stops.
 
 === 4 Host Program support