Document option in README.md.

... macro with LINALG_HAS_EXECUTION.
In the next commit, we'll conditionally define it
in a common place.

Make sure that LINALG_ENABLE_TBB is defined as a macro,
if set as a CMake option.  It's conditionally defined
in the generated header file linalg_config.h
(which already exists and conditionally defines other macros).

Also, make sure that macros.hpp includes the generated
header file linalg_config.h.
Given that linalg_config.h already had macros defined in it,
other stdBLAS headers really should have been including it.

Customization point execpolicy_mapper remains the same,
but algorithms now use it in a wrapper that checks the return type,
to ensure that infinite recursion errors are checked at compile time.

Also, fix <execution> header include guard.

Use ranges (views::iota, optionally also views::cartesian_product)
to make the algorithm work for any mdspan with unique layout.