Makefile tweaks to make packaging easier

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.