Move C compiler warning (-W) flags from CFLAGS to AM_CFLAGS.

Move C compiler warning (-W) flags from CFLAGS to AM_CFLAGS.

C compiler warning options added by the "configure" script have until
now been put into CFLAGS.  However that option is supposed to be reserved
for the user under Automake rules, so move them to AM_CFLAGS.

Besides increased adherence to Automake rules, this is useful because
AM_CFLAGS can be overridden on a per-target (e.g. program or library)
basis, whereas CFLAGS cannot.  In turn, building the sFlow library (which
will be added in an upcoming commit) requires overriding the compiler
flags to avoid some warning messages.  (By modifying the warning flags, we
avoid the need to modify the sFlow library in any way.)