provide sensible defaults (-O2) for systems with no default CFLAGS
tested on lunox
note: this also, as a not unwelcome side effect, changes the
default flags used for builds from "-g -O2" to "-O2" on the
standard GNU installation (and "-O2 -pipe" on OpenBSD, I
think). More flags (e.g. debugging) can and will be added
later.
git-svn-id: svn://www.freewrt.org/trunk/freewrt@55 afb5a338-a214-0410-bd46-81f09a774fd1