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)

sys/unix/sysconf

@@ -42,6 +42,7 @@ MAXPLAYERS=10
 # certain compilation options.  See src/end.c and sys/unix/hints/* for
 # more information.
 GDBPATH=/usr/bin/gdb
+GREPPATH=/bin/grep
 # Values are priorities: 0 - do not use this method, 1 - low priority,
 # 2 - high priority.  Non-zero priority methods are tried in order.
 PANICTRACE_GDB=1