Make check failure for generic (GEN) SIMD mode

[smangham]

I've recently tried to compile Grid on a desktop in the most simple, stripped-down way I could figure:

../configure --enable-precision=double --enable-comms=none

With the only libraries being GMP & MPFR, and using the default GEN SIMD mode as I didn't pass an argument.

First, this results in failure due to missing Version.h as per issue #227, but once that's fixed make check fails at testing double->half precision (tests/Test_simd.cc:787). The error is due to an assert(0) thrown in Grid/simd/Grid_generic.h:295, due to the define USE_FP16 not being set.

It looks like USE_FP16 is only defined for other SIMD modes (Grid_qpx.h:378, Grid_avx.h:464, Grid_avx512.h), but this test appears to be run regardless of configured SIMD mode.

Talking to a current user it seems this is known behaviour, so I'm just opening an issue to ensure it's documented.