Support for set_arg on macOS

[henryiii]

Currently, macOS does not support set_arg with some datatypes. This was partially fixed in #737, but it does not work if the user does not manually specify the <boost/compute/types.hpp> header manually.

This PR is expected to fail to build on macOS on the first build. It will then need to be fixed.

See ddemidov/vexcl#227 for more info.

[coveralls]

Coverage remained the same at 83.766% when pulling c01065e on henryiii:add_test_mactypes into 7daca6f on boostorg:develop.

[ddemidov]

@henryiii , can you try to add ddemidov@8f77efd to this PR? I believe that should fix the issue.

[coveralls]

Coverage remained the same at 83.766% when pulling 70aa256 on henryiii:add_test_mactypes into 7daca6f on boostorg:develop.

[ddemidov]

I think the problem now is that test_kernels.cpp does not include compute/core.hpp. And we can not include types.hpp into the kernel.hpp, because types.hpp specializes some templates only declared in kernel.hpp. So may be the best solution for the failing test would be to either include core.hpp instead of kernel.hpp, or include types.hpp after kernel.hpp.

[henryiii]

It bothers me that include order matters. Clang can even warn if includes are not sorted in alphabetical order.

Couldn't types include kernel, since it works on the contents?

Couldn't the types it specializes be declared first?

[coveralls]

Coverage remained the same at 84.02% when pulling 4ab0939 on henryiii:add_test_mactypes into 827371a on boostorg:develop.

[henryiii]

I believe this fix is okay to add.