[th/fix-hash-combine-bool] fix NM_HASH_COMBINE_BOOLS() macro with older clang

the recent rework of NM_HASH_COMBINE_BOOLS() broke compilation with clang 3.4.2-9.el7 (Centos7).

fix it, by reworking the implementation of the macro. It's actually better now, because the new form can be used as a compile time expression itself.

Also improve NM_STATIC_ASSERT_EXRP*() macros.