allow @ instead of @= in external compilers

lib/spack/spack/compilers/__init__.py

@@ -157,8 +157,9 @@ def _compiler_config_from_package_config(config):
 
 def _compiler_config_from_external(config):
     spec = spack.spec.Spec(config["spec"])
+    # use str(spec.versions) to allow `@x.y.z` instead of `@=x.y.z`
     compiler_spec = spack.spec.CompilerSpec(
-        package_name_to_compiler_name.get(spec.name, spec.name), spec.version
+        package_name_to_compiler_name.get(spec.name, spec.name), str(spec.versions)
     )
 
     extra_attributes = config.get("extra_attributes", {})

lib/spack/spack/test/cmd/compiler.py

@@ -300,7 +300,7 @@ def test_compilers_shows_packages_yaml_minimal(no_compilers_yaml, working_env, c
     gcc_entry = {
         "externals": [
             {
-                "spec": "gcc@=7.7.7",
+                "spec": "gcc@7.7.7",
                 "prefix": str(compilers_dir),
                 "modules": ["gcc/7.7.7", "foobar"],
                 "extra_attributes": {"flags": {"fflags": "-ffree-form"}},