gcc-8.1 compilation issue after #19 merged

[KineticTheory]

After #19 was merged, we are seeing the following gcc-8.1 compile error:

In file included from /home/kellyt/draco/src/experimental/mdspan:72,
                 from /home/kellyt/draco/src/experimental/test/tst_mdspan.cc:12:
/home/kellyt/draco/src/experimental/__p0009_bits/layout_stride.hpp:206:1: error: embedding a directive within macro arguments is not portable [-Werror]
 #if defined(__INTEL_COMPILER)
 ^
/home/kellyt/draco/src/experimental/__p0009_bits/layout_stride.hpp:209:1: error: embedding a directive within macro arguments is not portable [-Werror]
 #else
 ^
/home/kellyt/draco/src/experimental/__p0009_bits/layout_stride.hpp:211:1: error: embedding a directive within macro arguments is not portable [-Werror]
 #endif
 ^

• On a positive note, Numerous fixes and updates #19 resolved a fatal build error when we attempt to use intel-17.0.4.

cc: @brryan