linux: Makefile: add CPPFLAGS variable

[ghost]

This allows to set DEBUG_LOG from shell.

[burito]

CPPFLAGS is for C++, the appropriate C variable is CFLAGS

[ghost]

CPPFLAGS is for the C preprocessor. CXXFLAGS is to C++ what CFLAGS is to C.

https://www.gnu.org/software/make/manual/html_node/Implicit-Variables.html

[simon-wh]

Thank you! Those changes make a lot of sense, we're going to update the readme slightly to mention the ability to add the flag to get a bit of debug logging.

[ghost]

Awesome, thanks! :)