Leave --march unset by default (#1101)

install.py

@@ -341,7 +341,6 @@ def validate_path(path):
     "Debug" if debug else "RelWithDebInfo" if debug_release else "Release"
 )}
 -DBUILD_SHARED_LIBS=ON
--DBUILD_MARCH={str(march)}
 -DCMAKE_CUDA_ARCHITECTURES={str(arch)}
 -DLegion_MAX_DIM={str(maxdim)}
 -DLegion_MAX_FIELDS={str(maxfields)}
@@ -354,6 +353,8 @@ def validate_path(path):
 -DLegion_USE_HDF5={("ON" if hdf else "OFF")}
 """.splitlines()
 
+    if march:
+        cmake_flags += [f"-DBUILD_MARCH={march}"]
     if cuda_dir:
         cmake_flags += ["-DCUDAToolkit_ROOT=%s" % cuda_dir]
     if nccl_dir:
@@ -557,7 +558,7 @@ def driver():
         "--march",
         dest="march",
         required=False,
-        default=("haswell" if platform.machine() == "x86_64" else "native"),
+        default=("haswell" if platform.machine() == "x86_64" else None),
         help="Specify the target CPU architecture.",
     )
     parser.add_argument(