single manual page, appropriately linked, since this removes the
decision of which page the (previously non-existent) sigmask.2
MLINK should point at.
Submitted by: will
so that all these makefiles can be used to build libc_r too.
Added .if ${LIB} == "c" tests to restrict man page builds to libc
to avoid needlessly building them with libc_r too.
Split libc Makefile into Makefile and Makefile.inc to allow the
libc_r Makefile to include Makefile.inc too.