Improve AVX2 compiler support detection.

Commit 735c4070 added uses of intrinsics that require at least
 gcc 9.0 (cf. <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78782>),
 even though AVX2 support may appear to be available in earlier gcc
 versions.
We were not testing for this.
Update the compiler test in configure.ac to use these intrinsics
 explicitly, so it will error out and disable AVX2 if they are not
 available.