openmpi: add two_level_namespace variant for MacOS (#47202)

* Add two_level_namespace variant (default is disabled) for MacOS to enable building
executables and libraries with two level namespace enabled.
* Addressed reviewer comments.
* Moved two_level_namespace variant ahead of the patch that uses that variant to
get concretize to work properly.
* Removed extra print statements

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

@@ -575,6 +575,24 @@ class Openmpi(AutotoolsPackage, CudaPackage):
     variant("openshmem", default=False, description="Enable building OpenSHMEM")
     variant("debug", default=False, description="Make debug build", when="build_system=autotools")
 
+    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'.""",
+    )
+
+    # Patch to allow two-level namespace on a MacOS platform when building
+    # openmpi. Unfortuntately, the openmpi configure command has flat namespace
+    # hardwired in. In spack, this only works for openmpi up to versions 4,
+    # because for versions 5+ autoreconf is triggered (see below) and this
+    # patch needs to be applied (again) AFTER autoreconf ran.
+    @when("+two_level_namespace platform=darwin")
+    def patch(self):
+        filter_file(r"-flat_namespace", "-commons,use_dylibs", "configure")
+
     provides("mpi@:2.0", when="@:1.2")
     provides("mpi@:2.1", when="@1.3:1.7.2")
     provides("mpi@:2.2", when="@1.7.3:1.7.4")
@@ -997,11 +1015,15 @@ def die_without_fortran(self):
     def autoreconf(self, spec, prefix):
         perl = which("perl")
         perl("autogen.pl")
+        if spec.satisfies("+two_level_namespace platform=darwin"):
+            filter_file(r"-flat_namespace", "-commons,use_dylibs", "configure")
 
     @when("@5.0.0:5.0.1")
     def autoreconf(self, spec, prefix):
         perl = which("perl")
         perl("autogen.pl", "--force")
+        if spec.satisfies("+two_level_namespace platform=darwin"):
+            filter_file(r"-flat_namespace", "-commons,use_dylibs", "configure")
 
     def configure_args(self):
         spec = self.spec