httping: don't override optimization level, correctly disable debug support

The httping makefile by default appends -O3 to CFLAGS, overriding the
optimization flags set in Buildroot, so pass OFLAGS= to disable this.

The DEBUG=no argument has to be passed on the make cmdline and not in the
environment to take effect, as it is unconditionally set in the Makefile.
Notice that it shouldn't be passed to 'make install', as that otherwise
will try to run the host strip program.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>