build: pre-generate built headers in configure

zebra.h includes route_types.h, which means almost all of our Makefile
targets have a dependency on route_types.h.  While BUILT_SOURCES ensures
that this dependency is fulfilled on a "make all", this doesn't work
when building specific targets (e.g. "make lib/libfrr.la").

There seems to be no good way to add a dependency on everything, so the
best fix for this is to just generate the file during ./configure.

(The Makefile rule is still there, so if route_types.txt is changed,
route_types.h will be rebuilt.)

This only affects "initial" builds from a clean directory and/or builds
with --disable-dependency-tracking;  other than that gcc will output
Makefile dependencies during compilation so that route_types.h will
become a dependency for all output files.

gitversion.h has the same problem on a smaller scope.

command_lex.h & command_parse.h *don't* have the problem because they're
only used in specific files, and these files have the dependency
explicitly listed in the Makefile.  Hence, they can also be removed from
BUILT_SOURCES.

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>

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