Resolve xtl in user cmake

[AntoinePrv]

Instead of having the packager (Debian, Conda recipe) decide on xtl support, let the final user decide on wheter to use it.

The current behaviour is maintained and overridable until xsim 15.
In xsimd 15, the configure time option has no effect for installed targets.

Close #1338

[AntoinePrv]

@drew-parsons does that work for you?

[serge-sans-paille]

LGTM too, thanks

[drew-parsons]

I'm not certain how to judge this PR. It explicitly preserves the current behaviour of xsimd 14, i.e. reproduces the "inconsistent" behaviour that I reported, adding -DXSIMD_ENABLE_XTL_COMPLEX=1 with or without ENABLE_XTL_COMPLEX. I guess it will become more clear once xsimd 15 is released whether this patch works the way that I was thinking.

edit: wait, no, I think it's working fine for me. I didn't have the tests in my demo test organised as clearly as it could be. The "2 tests" it reported were for type float vs complex, not test_load_store vs test_load_store_xtl. doctest didn't report the SUBCASES clearly. Placing test_load_store_xtl in a separate TEST_CASE_TEMPLATE rather than a SUBCASE shows the patch is working. Compile flag -DXSIMD_ENABLE_XTL_COMPLEX=1 gets added by default in my configuration, but can be deactivated by configuring cmake -DXSIMD_ENABLE_XTL_COMPLEX=Off ... I'll update my bug report with the updated demo test.

So, looks like this patch is working fine with xsimd 14.

If I understood right, the ENABLE_XTL_COMPLEX handling for tests at

xsimd/test/CMakeLists.txt

Line 62 in 510b4f1

if (ENABLE_XTL_COMPLEX)

could be removed or refactored in the future with xsimd 15.

[drew-parsons]

Incidentally, I added an extra test run to the debian CI tests, "test xsimd-test-float" (and test "xsimd-test-clang-float"), which runs the tests with this patch with an explicit -DXSIMD_ENABLE_XTL_COMPLEX=Off. You can see in the test logs at https://ci.debian.net/packages/x/xsimd/testing/amd64/71383346/#S8
that it causes the -DXSIMD_ENABLE_XTL_COMPLEX=1 to be dropped from compile flags, as intended from this PR.