Require xtl when xsimd is found xsimd in Debian is built with -DENABLE_XTL_COMPLEX=ON, which makes the upstream build system require the "xtl" cmake target when using the "xsimd" cmake target. The problem is that the cmake config bits of xsimd do not have this requirement (due to ENABLE_XTL_COMPLEX=ON) anywhere, so simply looking for "xsimd" in cmake to use it gives a "xsimd" target that will try to link to a "xtl" library (since there is no "xtl" target available. To workaround this situation, if xsimd was found, then require also xtl: this will make sure there is a "xtl" cmake target. . This actually is a bug in the cmake config bits of xsimd: either setting ENABLE_XTL_COMPLEX=ON should not persist in the installed cmake bits, or the bits must remember ENABLE_XTL_COMPLEX=ON and require xtl.
