No changes, just move the makefile header to the base class.

It seems to make more sense to output the header comment in the base class as
it's not gnu-toolset-specific at all.
Add support for builddir variable to "gnu" toolset.

Allow running "make builddir=some-path" to create all the build files in the
specified directory instead of creating them directly in the source directory
as is done by default.
Merge pull request #23 from vslavik/gnu-builddir

Allow specifying builddir with gnu-toolset-produced makefiles