Makefile: allow environment to overide CFLAGS/compiler

Signed-off-by: Steven Noonan <steven@uplinklabs.net>
common.mk: drop unneeded QUIET_* variables

Signed-off-by: Steven Noonan <steven@uplinklabs.net>
libtime.a: don't include .cflags in the AR archive

Signed-off-by: Steven Noonan <steven@uplinklabs.net>
Makefile: add install target

Signed-off-by: Steven Noonan <steven@uplinklabs.net>