liquid-dsp: pass the supported x86 CPU extensions

By default, the liquid-dsp configure script looks at the current CPU
to find which x86 CPU extensions are supported. Those checks use
AC_TRY_RUN(), which cannot work in a cross-compilation environment,
and would anyway be wrong.

To fix this, we explicitly pass the appropriate ac_cv_have_<foo>_ext
autoconf cache variables.

Fixes:

  http://autobuild.buildroot.org/results/eb0/eb002463ed82de461fae11211415c07883a95386/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>