[qfcc] Allow extern builtin function declarations

build_builtin_function does the right thing, and it was only legacy
syntax functions that were affected anyway. Certainly, external
variables should not be initialized, but klik uses @extern { } wrapped
around several builtin functions and I had added the feature to allow
just this as it is rather convenient.

tools/qfcc/source/qc-parse.y

@@ -1093,9 +1093,6 @@ overloaded_identifier
 non_code_func
 	: '=' '#' expr
 		{
-			if ($<spec>-1.storage == sc_extern) {
-				error (0, "initializing external variable");
-			}
 			build_builtin_function ($<symbol>0, $3, 0, $<spec>-1.storage);
 		}
 	| '=' expr