FindMPI: Honor MPI_CXX_SKIP_MPICXX when set as a normal variable

Express the cache entry with `option()`.  In combination with policies
CMP0077 and CMP0102, this allows project code to control the switch with
a variable:

```
set(MPI_CXX_SKIP_MPICXX TRUE)
find_package(MPI COMPONENTS CXX)
```