Explicitly set search paths for libraries build.

The libraries build is prepared to work with binaries not part of the current
working tree. But in this case the default search path surely points to some
other working tree. Even the binaries in this working tree might have been
compiled with non-standard builtin search paths.

Anyway when building the libraries we want always to use the headers from
the current working tree. Therefore we want to set them explicitly. Instead
of setting all in all five paths on the command lines of ca65, cc65 and ld65 I
opted to just set the single environment variable.