kconfig: add CC_IS_GCC and GCC_VERSION

This will be useful to specify the required compiler version,
like this:

config FOO
        bool "Use Foo"
        depends on GCC_VERSION >= 40800
        help
          This feature requires GCC 4.8 or newer.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Kees Cook <keescook@chromium.org>

init/Kconfig

@@ -8,6 +8,14 @@ config DEFCONFIG_LIST
 	default ARCH_DEFCONFIG
 	default "arch/$(ARCH)/defconfig"
 
+config CC_IS_GCC
+	def_bool $(success,$(CC) --version | head -n 1 | grep -q gcc)
+
+config GCC_VERSION
+	int
+	default $(shell,$(srctree)/scripts/gcc-version.sh -p $(CC) | sed 's/^0*//') if CC_IS_GCC
+	default 0
+
 config CONSTRUCTORS
 	bool
 	depends on !UML