kbuild: compile-test kernel headers to ensure they are self-contained

The headers in include/ are globally used in the kernel source tree
to provide common APIs. They are included from external modules, too.

It will be useful to make as many headers self-contained as possible
so that we do not have to rely on a specific include order.

There are more than 4000 headers in include/. In my rough analysis,
70% of them are already self-contained. With efforts, most of them
can be self-contained.

For now, we must exclude more than 1000 headers just because they
cannot be compiled as standalone units. I added them to header-test-.
The blacklist was mostly generated by a script, so the reason of the
breakage should be checked later.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Tested-by: Jani Nikula <jani.nikula@intel.com>
Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>

Makefile

@@ -618,6 +618,7 @@ ifeq ($(KBUILD_EXTMOD),)
 init-y		:= init/
 drivers-y	:= drivers/ sound/
 drivers-$(CONFIG_SAMPLES) += samples/
+drivers-$(CONFIG_KERNEL_HEADER_TEST) += include/
 net-y		:= net/
 libs-y		:= lib/
 core-y		:= usr/

init/Kconfig

@@ -108,6 +108,17 @@ config HEADER_TEST
 	  If you are a developer or tester and want to ensure the requested
 	  headers are self-contained, say Y here. Otherwise, choose N.
 
+config KERNEL_HEADER_TEST
+	bool "Compile test kernel headers"
+	depends on HEADER_TEST
+	help
+	  Headers in include/ are used to build external moduls.
+	  Compile test them to ensure they are self-contained, i.e.
+	  compilable as standalone units.
+
+	  If you are a developer or tester and want to ensure the headers
+	  in include/ are self-contained, say Y here. Otherwise, choose N.
+
 config UAPI_HEADER_TEST
 	bool "Compile test UAPI headers"
 	depends on HEADER_TEST && HEADERS_INSTALL && CC_CAN_LINK