[configure/cleanup] fix broken autoconf cache variables

[configure/cleanup] fix broken autoconf cache variables

Fix zebra_rtread and zebra_ipforward_path.

It looks like someone tried to make this cached, but it was being ignored at
least on autoconf 2.6.1.  2.6.2 now gives a warning about the situation,
hence this fix.

Although they are just warnings, it's dangerous to leave them as-is, because
someone not diligent in reading the autoconf manual might just stick _cv_ in
the name, with the effect that running a ./configure --config-cache would
fail horribly, since variables are being set inside the AC_CACHE_CHECK()
block that would not be seen when the variable's value was read from cache.

Also added m4 quotes [] to macro arguments as autoconf manual
suggests.