Add variant for the --enable-two-level-namespace option in MPICH (#27230)

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

@@ -81,6 +81,15 @@ class Mpich(AutotoolsPackage):
             description='Enable Argobots support')
     variant('fortran', default=True, description='Enable Fortran support')
 
+    variant(
+        'two_level_namespace',
+        default=False,
+        description='''Build shared libraries and programs
+built with the mpicc/mpifort/etc. compiler wrappers
+with '-Wl,-commons,use_dylibs' and without
+'-Wl,-flat_namespace'.'''
+    )
+
     provides('mpi@:3.1')
     provides('mpi@:3.0', when='@:3.1')
     provides('mpi@:2.2', when='@:1.2')
@@ -475,6 +484,9 @@ def configure_args(self):
             config_args.append('--with-thread-package=argobots')
             config_args.append('--with-argobots=' + spec['argobots'].prefix)
 
+        if '+two_level_namespace' in spec:
+            config_args.append('--enable-two-level-namespace')
+
         return config_args
 
     @run_after('install')