make grep's path adjustable for PANICTRACE + more bits

Mac OS X doesn't have /bin/grep, only /usr/bin/grep.  Grr.
Don't try to parse PANICTRACE_GLIBC if the field doesn't exist.

sys/unix/hints/macosx10.5

@@ -64,6 +64,8 @@ CFLAGS=-g -I../include
 CFLAGS+=-DNOCLIPPING -DNOMAIL -DNOTPARMDECL -DHACKDIR=\"$(HACKDIR)\"
 CFLAGS+= -DDEFAULT_WINDOW_SYS=\"$(WANT_DEFAULT)\" -DDLB
 
+CFLAGS+= -DGREPPATH=\"/usr/bin/grep\"
+
 ifdef WANT_WIN_TTY
 WINSRC = $(WINTTYSRC)
 WINOBJ = $(WINTTYOBJ)