package/Makefile.in: Do not use CPPFLAGS for hardening options

package/Makefile.in: Do not use CPPFLAGS for hardening options

The hardening options are compiler flags, not pure pre-processor flags, so
put them in CFLAGS, not CPPFLAGS.

This fixes build errors where -D_FORTIFY_SOURCE=2 whas put in CPPFLAGS and
then applied to configure tests which could fail since the required -O2 is
only in CFLAGS.

Originally submitted as
http://patchwork.ozlabs.org/patch/904057/

Signed-off-by: Stefan Sørensen <stefan.sorensen@spectralink.com>
Signed-off-by: Matthew Weber <matthew.weber@rockwellcollins.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>