Explicit negation of feature with ./wcb --with-*=no for Spack ~* variants

[brettviren]

It occurs to me that a potential bug can exist in the current WCT Spack recipe.

The user may not enable optional variants (eg no +hdf) or may explicitly disable (eg ~hdf) and this can still cause ./wcb configure to find some installation. For example if the build host has HDF5 OS packages installed, then wcb can find them "for us" via pkg-config.

What is needed is to remove ambiguity and explicitly turn off wcb build config options that were not explicitly turned on by a Spack variant.