The ntpd package ships autoconf macros which attempt to test for the C99
compliance of `snprintf()` and `vsnprintf()`.

Those macros unconditionally consider the functions not compliant in a cross
compile setting and therfore break all subsequent configure tests because
the `snprintf` and `vsnprintf` symbols are shadowed with `#define` directives
redirecting them to their respectiv gnulib replacements.

The fortify source headers in turn will `#undef` those defines when wrapping
the associated functions and therfore cause all `conftest` programs to emit
a failure.

Pass the required magic autoconf cache variables via `CONFIGURE_VARS` to let
`./configure` assume C99 compliance of the affected functions, thus fixing
the build.

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>