Switch mpc to overlay convention

.orchestra/config/components.yml

 #@ load("@ytt:template", "template")
 #@ load("@ytt:data", "data")
 
-#@ load("/components/mpc.lib.yml", "mpc_component")
 #@ load("/components/mpfr.lib.yml", "mpfr_component")
 #@ load("/components/cmake.lib.yml", "cmake_component")
 #@ load("/components/zlib.lib.yml", "zlib_component")
@@ -53,7 +52,6 @@
 
 #@ load("/global_options.lib.yml", "options")
 components:
-  mpc: #@ mpc_component
   mpfr: #@ mpfr_component
   cmake: #@ cmake_component
   zlib: #@ zlib_component

.orchestra/config/components/mpc.lib.yml → .orchestra/config/components/mpc.yml

+#@ load("@ytt:overlay", "overlay")
 #@ load("/lib/create_component.lib.yml", "single_build_component")
 
 #@ source_url = "https://ftp.gnu.org/gnu/mpc//mpc-1.1.0.tar.gz"
@@ -19,6 +20,8 @@ dependencies:
   - mpfr
 #@ end
 
+#@overlay/match by=overlay.all, expects=1
+#@overlay/match-child-defaults missing_ok=True
 ---
-
-#@ mpc_component = single_build_component(**_mpc_args())
+components:
+  mpc: #@ single_build_component(**_mpc_args())