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>

It's a problem about cscope target of kernel Makefile, and the cscope
plugin of emacs:
1. `make cscope` will generate cscope.files cscope.{in,po,}.out;
2. the cscope plugin expect a cscope.out.{in,po,};
3. the default `cscope -b` would generate cscope.{in,po,}.out;

There are three approach to solve it:
1. modify the cscope C code;
2. modify the cscope emacs plugin lisp code;
3. modify the Makefile;

I have tried to communicate with the cscope upstream, but later I
realize the third approach is most meaningful.

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