unifdef: count lines correctly when processing multiple files
Makefile: need a dependency on unifdef.h
reversion: avoid spurious rebuilds in a dirty tree
unifdef: Fix the behaviour of #define without a value

On the command line, `-DFOO` is a short-hand for `-DFOO=1`
whereas `#define FOO` is like `-DFOO=`.
unifdef: more refactoring in -f mode

Moving towards sharing more of the parsing code between -f header
files and normal source files.
tests: display progress
unifdef: 64 bit evaluator, and quell type conversion warnings
unifdef: rewrite the #define/#undef file parser