Add CPPFLAGS, LDFLAGS to gcc build and harden

CPPFLAGS is a typical way to add -D_FORTIFY_SOURCE=2

-fPIE and -fstack-protector-all are added to CDEBUGFLAGS

LDFLAGS allows us to use -pie -Wl,-z,relro -Wl,-z,now
hardening options as well as -Wl,-O1 -Wl,--sort-common
linker optimisations.