where is this `LINALG_ENABLE_KOKKOS_DEFAULT` set?

@crtrott @mhoemmen

I was trying to build using Kokkos, using this cmake line:
when I configure, I use:

    cmake \
	-Dmdspan_DIR=${MDSPAN_PFX}/lib/cmake/mdspan \
	-DLINALG_ENABLE_TESTS=On \
	-DLINALG_ENABLE_EXAMPLES=On \
	-DCMAKE_BUILD_TYPE="${btype}" \
	\
	-DLINALG_ENABLE_KOKKOS=On \
	-DKokkos_DIR=${KOKKOS_DIR} \
	-DKokkosKernels_DIR=${KOKKOS_KER_DIR} \
	\
	-DCMAKE_VERBOSE_MAKEFILE=On \
	-DCMAKE_INSTALL_PREFIX=../stdblas-install \
	-DGTEST_ROOT=../install_gtest \
	../stdBLAS/

But that causes a problem, because in this file: linalg_execpolicy_mapper.hpp we have:

#if defined(LINALG_ENABLE_KOKKOS) && defined(LINALG_ENABLE_KOKKOS_DEFAULT)
#include <experimental/__p1673_bits/kokkos-kernels/exec_policy_wrapper_kk.hpp>
#endif

and since LINALG_ENABLE_KOKKOS_DEFAULT is NOT set at all, I get a compile error because that header is not included.
If I change that to || it works.

But was wondering: what is the role of LINALG_ENABLE_KOKKOS_DEFAULT? Is that supposed to be set by user? Or what?

Start by inspecting linalg_execpolicy_mapper.hpp and the CMake configuration used in the report to trace whether LINALG_ENABLE_KOKKOS_DEFAULT is defined or propagated. Reproduce the reported Kokkos configuration and compile failure, then document or correct the expected macro behavior so the Kokkos build completes without changing the condition ad hoc.