Honor the value of CPPFLAGS set in the environment

[rlaboiss]

This in necessary for distributions, like Debian, that require that C/C++ code must be compiled with certain options (like -D_FORTIFY_SOURCE=2, see the Hardening Requirements in Debian).

In the present patch, the statements for setting the variables CPPFLAGS in the Makefiles have been changed from = to +=.

[adriaandegroot]

Are you intentionally not touching external/flac and external/gsl ? (Or perhaps asked differently: doesn't Debian use unbundled dependencies, so why bother changing the external/ bits?)

[rlaboiss]

(Or perhaps asked differently: doesn't Debian use unbundled dependencies, so why bother changing the external/ bits?)

No, Debian uses the bundled flac and gsl dependencies.

Are you intentionally not touching external/flac and external/gsl ?

Thanks for drawing my attention to this. Debian has a mechanism for injecting the hardening flags into the generated file makefile.defs (the debian/rules file does it through a snippet). Since makefile.defs is included in flac/Makefile and gsl/Makefile, there is no need for patching them. On the other hand, the patch is necessary for the other Makefiles because they do not respect previous settings of CPPFLAGS.