try for system libmgba first

CMake/FindLIBMGBA.cmake

@@ -0,0 +1,15 @@
+find_path(LIBMGBA_INCLUDE_DIR flags.h PATH_SUFFIXES mgba)
+find_library(LIBMGBA_LIBRARY mgba)
+mark_as_advanced(LIBMGBA_INCLUDE_DIR LIBMGBA_LIBRARY)
+
+include(FindPackageHandleStandardArgs)
+find_package_handle_standard_args(LIBMGBA DEFAULT_MSG
+	LIBMGBA_INCLUDE_DIR LIBMGBA_LIBRARY)
+
+if(LIBMGBA_FOUND AND NOT mGBA::mgba)
+  add_library(mGBA::mgba UNKNOWN IMPORTED)
+  set_target_properties(mGBA::mgba PROPERTIES
+    IMPORTED_LOCATION "${LIBMGBA_LIBRARY}"
+    INTERFACE_INCLUDE_DIRECTORIES "${LIBMGBA_INCLUDE_DIR}"
+  )
+endif()

CMakeLists.txt

@@ -944,9 +944,12 @@ if(NOT ENABLE_QT)
   set(USE_MGBA 0)
 endif()
 if(USE_MGBA)
+  find_package(LIBMGBA)
+  if(NOT LIBMGBA_FOUND)
     message(STATUS "Using static libmgba from Externals")
     add_subdirectory(Externals/mGBA)
   endif()
+endif()
 
 find_package(SYSTEMD)
 if(SYSTEMD_FOUND)