Fix logic bug and make language explicit

The operator precedence in CMake can be surprising, so add parenthesis
to make in unambigious. Also, make it more explicit from which language
we get the default options. (They are all set from the settings given
for C++; why would have different optimization options for C and C++?)

cmake/Modules/UseDebugSymbols.cmake

@@ -16,7 +16,7 @@ is_compiler_gcc_compatible ()
 # only debugging using the GNU toolchain is supported for now
 if (CXX_COMPAT_GCC)
   # default debug level, if not specified by the user
-  set_default_option (_dbg_flag "-ggdb3" "(^|\ )-g")
+  set_default_option (CXX _dbg_flag "-ggdb3" "(^|\ )-g")
 
   # add debug symbols to *all* targets, regardless. there WILL come a
   # time when you need to find a bug which only manifests itself in a