Documentation: kconfig: add recommended way to describe compiler support

It would be nice if the source code is written in the same style.
This proposes the convention for describing the compiler capability
in Kconfig.

Signed-off-by: Masahiro Yamada <[e-mail]>

Documentation/kbuild/kconfig-language.txt

···
 The dependencies are moved to the symbol GENERIC_IOMAP and we avoid the
 situation where select forces a symbol equals to 'y'.
 
+Adding features that need compiler support
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+There are several features that need compiler support. The recommended way
+to describe the dependency on the compiler feature is to use "depends on"
+followed by a test macro.
+
+config CC_STACKPROTECTOR
+	bool "Stack Protector buffer overflow detection"
+	depends on $(cc-option,-fstack-protector)
+	...
+
+If you need to expose a compiler capability to makefiles and/or C source files,
+CC_HAS_ is the recommended prefix for the config option.
+
+config CC_HAS_STACKPROTECTOR_NONE
+	def_bool $(cc-option,-fno-stack-protector)
+
 Build as module only
 ~~~~~~~~~~~~~~~~~~~~
 To restrict a component build to module-only, qualify its config symbol