Mac OS X Lion: `externally-visible` causes zillions of warnings

[Werner LEMBERG]

Compiling emacs master (2024136d31e) on a Mac OS X Lion box with the
system's `/usr/bin/clang` compiler, there are many warnings like the
following.

```
In file included from dispnew.c:27:
./lisp.h:69:1: warning: unknown attribute 'externally_visible' ignored 
[-Wattributes]
DEFINE_GDB_SYMBOL_BEGIN (int, GCTYPEBITS)
^
./lisp.h:55:51: note: expanded from macro 'DEFINE_GDB_SYMBOL_BEGIN'
# define DEFINE_GDB_SYMBOL_BEGIN(type, id) extern DECLARE_GDB_SYM (type, id)
                                                  ^
./lisp.h:50:49: note: expanded from macro 'DECLARE_GDB_SYM'
#define DECLARE_GDB_SYM(type, id) type const id EXTERNALLY_VISIBLE
                                                ^
./conf_post.h:281:28: note: expanded from macro 'EXTERNALLY_VISIBLE'
#define EXTERNALLY_VISIBLE _GL_ATTRIBUTE_EXTERNALLY_VISIBLE
                           ^
./config.h:2542:59: note: expanded from macro '_GL_ATTRIBUTE_EXTERNALLY_VISIBLE'
# define _GL_ATTRIBUTE_EXTERNALLY_VISIBLE __attribute__ ((externally_visible))
```

As mentioned in another e-mail, this increases the log size by a
factor of 200.

`/usr/bin/clang` identifies itself as

```
Apple LLVM version 4.2 (clang-425.0.28) (based on LLVM 3.2svn)
Target: x86_64-apple-darwin11.4.2
Thread model: posix
```


    Werner