Leave GDB_COMMAND undefined if it's undefined

This way we can use the preprocessor to check when it's undefined, not
just the compiler.

Author: roystgnr

m4/libmesh_core_features.m4

@@ -19,8 +19,14 @@ AC_ARG_WITH([gdb-command],
     [gdb_command="$withval"],
     [gdb_command="no"])
 
-AC_DEFINE_UNQUOTED(GDB_COMMAND, "$gdb_command", [command to invoke gdb])
-AC_MSG_RESULT([configuring gdb command... "$gdb_command"])
+AS_IF([test "$gdb_command" != no],
+      [
+        AC_DEFINE_UNQUOTED(GDB_COMMAND, "$gdb_command", [command to invoke gdb])
+        AC_MSG_RESULT([configuring gdb command... "$gdb_command"])
+      ],
+      [
+        AC_MSG_RESULT([configuring with no gdb"])
+      ])
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