El Capitan system: always include GCC compiler (LLNL#515)

always include gcc in config for build deps that dont work with cce; add preference for cce when setting compiler=cce

systems/llnl-elcapitan/system.py

@@ -105,8 +105,30 @@ def external_pkg_configs(self):
         elif self.spec.satisfies("compiler=gcc"):
             selections.append(externals / "libsci" / "00-gcc-packages.yaml")
 
+        cmp_preference_path = self.next_adhoc_cfg()
+        with open(cmp_preference_path, "w") as f:
+            f.write(self.compiler_weighting_cfg())
+        selections.append(cmp_preference_path)
+
         return selections
 
+    def compiler_weighting_cfg(self):
+        compiler = self.spec.variants["compiler"][0]
+
+        if compiler == "cce":
+            return """\
+packages:
+  all:
+    require:
+    - one_of: ["%cce", "@:"]
+"""
+        elif compiler == "gcc":
+            return """\
+packages: {}
+"""
+        else:
+            raise ValueError(f"Unexpected value for compiler: {compiler}")
+
     def compiler_configs(self):
         compilers = LlnlElcapitan.resource_location / "compilers"
 
@@ -118,8 +140,10 @@ def compiler_configs(self):
                     self.rocm_cce_compiler_cfg(self.spec.variants["rocm"][0], "16.0.0")
                 )
             selections.append(compiler_cfg_path)
-        elif self.spec.satisfies("compiler=gcc"):
-            selections.append(compilers / "gcc" / "00-gcc-12-compilers.yaml")
+
+        # Note: this is always included for some low-level dependencies
+        # that shouldn't build with %cce
+        selections.append(compilers / "gcc" / "00-gcc-12-compilers.yaml")
 
         return selections