share/mk/: build-book, build-man, install-man: Specify version and date

share/mk/: build-book, build-man, install-man: Specify version and date

Each page has its own date.

Add new 'build-man' target, which stamps the date and version in the
pages (in $builddir, not the source ones).

Build-system internal stuff:
	$CURDIR will be used for the build system, while $srcdir will be
	used for the project root of the repository.  This helps use
	this build system in other projects.

Signed-off-by: Alejandro Colomar <alx@kernel.org>