Explicitly create output directories for flex/yacc output.

When out-of-source build with '--disable-dependency-tracking' is
performed, the 'lib/route' and 'lib/route/cls' directories required for
flex/yacc output do not exist. As a result, the flex/yacc calls fail
with ENOENT.

Create the necessary directories explicitly via $(MKDIR_P)
in the flex/yacc rules to guarantee proper out-of-source and in-source
build.