Makefile tweaks to make packaging easier

[mutability]

Two changes to support Debian's hardening mechanism, which would like to provide extra CFLAGS/CPPFLAGS when building:

• Add to CFLAGS, rather than overriding its value
• Include CPPFLAGS (preprocessor flags) when compiling

And one change to remove an unnecessary library dependency:

• Don't link view1090 to librtlsdr, it doesn't use it.