Make mpich buildable without fortran (#17964)

When the user explicitly sets ~fortran, mpich builds without fortran
support. This will make building C/C++ libraries using clang easier,
since clang does not offer a fortran compiler by default (yet).

Since the user has to disable Fortran support explicitly, this change
is not breaking.

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

@@ -69,6 +69,7 @@ class Mpich(AutotoolsPackage):
                         'minimalistic implementation')
     variant('argobots', default=False,
             description='Enable Argobots support')
+    variant('fortran', default=True, description='Enable Fortran support')
 
     provides('mpi')
     provides('mpi@:3.0', when='@3:')
@@ -209,21 +210,25 @@ def setup_dependent_build_environment(self, env, dependent_spec):
         env.set('MPICH_FC', spack_fc)
 
     def setup_dependent_package(self, module, dependent_spec):
+        spec = self.spec
+
         # For Cray MPIs, the regular compiler wrappers *are* the MPI wrappers.
         # Cray MPIs always have cray in the module name, e.g. "cray-mpich"
-        external_modules = self.spec.external_modules
+        external_modules = spec.external_modules
         if external_modules and 'cray' in external_modules[0]:
-            self.spec.mpicc = spack_cc
-            self.spec.mpicxx = spack_cxx
-            self.spec.mpifc = spack_fc
-            self.spec.mpif77 = spack_f77
+            spec.mpicc = spack_cc
+            spec.mpicxx = spack_cxx
+            spec.mpifc = spack_fc
+            spec.mpif77 = spack_f77
         else:
-            self.spec.mpicc = join_path(self.prefix.bin, 'mpicc')
-            self.spec.mpicxx = join_path(self.prefix.bin, 'mpic++')
-            self.spec.mpifc = join_path(self.prefix.bin, 'mpif90')
-            self.spec.mpif77 = join_path(self.prefix.bin, 'mpif77')
+            spec.mpicc = join_path(self.prefix.bin, 'mpicc')
+            spec.mpicxx = join_path(self.prefix.bin, 'mpic++')
+
+            if '+fortran' in spec:
+                spec.mpifc = join_path(self.prefix.bin, 'mpif90')
+                spec.mpif77 = join_path(self.prefix.bin, 'mpif77')
 
-        self.spec.mpicxx_shared_libs = [
+        spec.mpicxx_shared_libs = [
             join_path(self.prefix.lib, 'libmpicxx.{0}'.format(dso_suffix)),
             join_path(self.prefix.lib, 'libmpi.{0}'.format(dso_suffix))
         ]
@@ -243,9 +248,18 @@ def die_without_fortran(self):
         # 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):
+        # The user can work around this by disabling Fortran explicitly
+        # with ~fortran
+
+        f77 = self.compiler.f77
+        fc = self.compiler.fc
+
+        fortran_missing = f77 is None or fc is None
+
+        if '+fortran' in self.spec and fortran_missing:
             raise InstallError(
-                'MPICH requires both C and Fortran compilers!'
+                'mpich +fortran requires Fortran compilers. Configure '
+                'Fortran compiler or disable Fortran support with ~fortran'
             )
 
     def configure_args(self):
@@ -262,6 +276,9 @@ def configure_args(self):
                                                 spec else 'yes')
         ]
 
+        if '~fortran' in spec:
+            config_args.append('--disable-fortran')
+
         if '+slurm' in spec:
             config_args.append('--with-slurm=yes')
             config_args.append('--with-slurm-include={0}'.format(