Local compiler flags ignored

[PeterBBBBB]

The Makefile ignores any local settings of compiler flags (hardening for example).

1. Flags set with CFLAGS= which overwrites. CFLAGS+= would add to existing values.
2. Any local linker flags in $LDFLAGS are not used at all.

Carrying a patch for this here
https://salsa.debian.org/debian/duma/-/blob/master/debian/patches/002-makefile.patch

C++ flags are usually in CXXFLAGS not CPPFLAGS (for the C preprocessor)
but this does not itself seem to cause a problem.