bsd.prog.mk -- we need to compile PIC, which requires a library build.
With this change, USDT (userspace DTrace probes) work from within
shared libraries.
PR: kern/159046
Submitted by: Alex Samorukov <samm at os2.kiev.ua>
Comments by: Scott Lystig Fritchie <slfritchie at snookles.com>
MFC after: 3 days