Now that make(1) passes variables that were given on the command line

Now that make(1) passes variables that were given on the command line
to subordinate make(1) invocations through MAKEFLAGS, we cannot add
CFLAGS onto the make(1) command line. This will conflict with the
individual makefiles wanting to append to it, which is not respected
when CFLAGS is given on the command line. Hence build breakage.
So, put CFLAGS in the environment instead.