configure: fix iOS builds
configure: fix iOS builds

possibly other cross-compiles; avoids misusing -mavx2/-msse2 in these
cases by checking the usability of the associated intrinsics header
files.

iosbuild.sh has been broken since at least:
6e61a3a configure: test for -msse2

Change-Id: Ic650d9eec70f6a5de7d89997b3b425a4aa50ccd9

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