require fortran for MPI providers to avoid delayed compiler errors (#1937)

* openmpi: require fortran for now

* mpich: requre fortran for now

* mvapich2: require fortran for now

* openmpi: flake8 fixes

var/spack/repos/builtin/packages/mpich/package.py

@@ -82,6 +82,13 @@ def setup_dependent_package(self, module, dep_spec):
         ]
 
     def install(self, spec, prefix):
+        # Until we can pass variants such as +fortran through virtual
+        # dependencies depends_on('mpi'), require Fortran compiler to
+        # avoid delayed build errors in dependents.
+        if (self.compiler.f77 is None) or (self.compiler.fc is None):
+            raise InstallError('Mpich requires both C and Fortran ',
+                               'compilers!')
+
         config_args = [
             '--prefix={0}'.format(prefix),
             '--enable-shared',