auto-set nvcc host compiler with FindMPI detection

auto-set nvcc host compiler with FindMPI detection

Distinguish between MPI build that use MPI compiler wrapper and FindMPI
as as in the latter case the CMAKE_C_COMPILER can be used safely as the
CUDA compiler's CPU backend.

Additional minor changes in cmake/gmxManageNvccConfig.cmake:
- added more detailed explanation on some strange things done in order
  to please nvcc;
- bumped icc gcc compatibility version to 4.5 with CUDA <=v4.1;
- added explanation on CUDA code generation;
- typos fixed.

Change-Id: Ia82bc63c129ac897ce88175f919b4f034e7ce665