docsrc: build Documentation/ sources

Currently source files in the Documentation/ sub-dir can easily bit-rot
since they are not generally buildable, either because they are hidden in
text files or because there are no Makefile rules for them.  This needs to
be fixed so that the source files remain usable and good examples of code
instead of bad examples.

Add the ability to build source files that are in the Documentation/ dir.
Add to Kconfig as "BUILD_DOCSRC" config symbol.

Use "CONFIG_BUILD_DOCSRC=1 make ..." to build objects from the
Documentation/ sources.  Or enable BUILD_DOCSRC in the *config system.
However, this symbol depends on HEADERS_CHECK since the header files need
to be installed (for userspace builds).

Built (using cross-tools) for x86-64, i386, alpha, ia64, sparc32,
sparc64, powerpc, sh, m68k, & mips.

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

lib/Kconfig.debug

@@ -735,6 +735,15 @@ config FIREWIRE_OHCI_REMOTE_DMA
 
 	  If unsure, say N.
 
+menuconfig BUILD_DOCSRC
+	bool "Build targets in Documentation/ tree"
+	depends on HEADERS_CHECK
+	help
+	  This option attempts to build objects from the source files in the
+	  kernel Documentation/ tree.
+
+	  Say N if you are unsure.
+
 source "samples/Kconfig"
 
 source "lib/Kconfig.kgdb"