This is a generalized solution to #15 and #29. It adds one preprocessor macro to the sketch file, while achieving the same effects without introducing special files:

#pragma arduino

Whatever text comes after this preprocessor macro, to the end of the line, is treated as part of the UserSettings.txt. For example putting the followign lines in the sketch:

#pragma arduino compiler.c.extra_flags=-DNDEBUG
#pragma arduino compiler.cpp.extra_flags={compiler.c.extra_flags} -DTESTLIBRARY_BUFSIZE=100

is equivalent to adding the following lines to UserSettings.txt for the specific sketch:

compiler.c.extra_flags=-DNDEBUG
compiler.cpp.extra_flags={compiler.c.extra_flags} -DTESTLIBRARY_BUFSIZE=100