Add an 'extern' attribute to struct!

So that it can be used to refer to the external variable in a library.
The format is:

make struct! [
	[extern: [lib "sym"]]
	;struct definition
]

example code:
rebol []

ncurses: make library! %libncursesw.so

stdscr: make struct! compose/deep [
	[
		extern: [(ncurses) "stdscr"]
	]
	ptr [pointer]
]

print ["stdscr:" stdscr/ptr]

close ncurses

src/boot/errors.r

@@ -190,6 +190,7 @@ Access: [
 	permission-denied: 	[{permission denied}]
 	process-not-found: 	[{process not found:} :arg1]
 
+	symbol-not-found:	[{symbol not found:} :arg1]
 ]
 
 Command: [

src/boot/words.r

@@ -190,6 +190,7 @@ pointer
 addr
 raw-memory
 raw-size
+extern
 rebval
 
 ;routine

src/core/t-struct.c

@@ -436,6 +436,37 @@ static void parse_attr (REBVAL *blk, REBINT *raw_size, REBUPT *raw_addr)
 						Trap_Arg(attr);
 					}
 					break;
+				case SYM_EXTERN:
+					++ attr;
+
+					if (*raw_addr != 0) /* raw-memory is exclusive with extern */
+						Trap_Arg(attr);
+
+					if (!IS_BLOCK(attr)
+						|| VAL_LEN(attr) != 2) {
+						Trap_Arg(attr);
+					} else {
+						REBVAL *lib;
+						REBVAL *sym;
+						void *addr;
+
+						lib = VAL_BLK_SKIP(attr, 0);
+						sym = VAL_BLK_SKIP(attr, 1);
+
+						if (!IS_LIBRARY(lib))
+							Trap_Arg(attr);
+						if (IS_CLOSED_LIB(VAL_LIB_HANDLE(lib)))
+							Trap0(RE_BAD_LIBRARY);
+						if (!ANY_BINSTR(sym))
+							Trap_Arg(sym);
+
+						addr = OS_FIND_FUNCTION(LIB_FD(VAL_LIB_HANDLE(lib)), VAL_DATA(sym));
+						if (!addr)
+							Trap1(RE_SYMBOL_NOT_FOUND, sym);
+
+						*raw_addr = (REBUPT)addr;
+					}
+					break;
 					/*
 					   case SYM_ALIGNMENT:
 					   ++ attr;