Proper way to determine place of system sources in makefile?

[Lev Serebryakov]

Hello, Hackers.

 I'm  writing  some code, which is built outside of system sources but
depends on them.

 I'm using FreeBSD mk infrastructure.

 When code is kernel module (uses <bsd.kmod.mk>) here is SYSDIR
variable.

  But which is proper way to refer to system sources when makefile is
prepared for shared library (<bsd.lib.mk>) or program (<bsd.prog.mk>)?

-- 
// Black Lion AKA Lev Serebryakov <lev at FreeBSD.org>