mk scripts: add flag for compiling with nbsd libc.
This patch changes the system mk scripts to enable compilation
of programs using the BSD make system to compile with the new
libc.
In details, it does the following:
- it always defines the __MINIX make variable. This can be used,
in porting applications, to specialize Makefiles for Minix.
- If the environment variable NBSD is set to something different
than 'no' and if the compiler is not ack, set NBSD_LIBC to 'yes'.
This will set the destination lib directory to '/usr/netbsd/lib'
and set up CPPFLAGS and LDFLAGS to use new libc's includes and
library directory.