ok, this is kind of messy.

first, we have /usr/include/netinet/in.h, which does:

#ifdef __OPTIMIZE__
[ ... ]
#  if __BYTE_ORDER == __LITTLE_ENDIAN
#   define ntohl(x)	__bswap_32 (x)
#   define ntohs(x)	__bswap_16 (x)
#   define htonl(x)	__bswap_32 (x)
#   define htons(x)	__bswap_16 (x)
#  endif
[ ... ]
#endif

then, in /usr/include/bits/byteswap.h, we get this:

#  define __bswap_32(x) \
     (__extension__							      \
      ({ register unsigned int __v;					      \
	 if (__builtin_constant_p (x))					      \
	   __v = __bswap_constant_32 (x);				      \
	 else								      \
	   __asm__ __volatile__ ("bswap %0"				      \
				 : "=r" (__v)				      \
				 : "0" ((unsigned int) (x)));		      \
	 __v; }))

that's the background.

take a small piece of c++ code that assigns a global scope variable using
htonl() and "houston, we have a problem."

[alane@wozzle /tmp]$ cat t.cc
#include <netinet/in.h>

uint32_t noaddr = htonl(INADDR_NONE);
[alane@wozzle /tmp]$ g++ -c t.cc
[alane@wozzle /tmp]$ g++ -O -c t.cc
t.cc:3: braced-group within expression allowed only inside a function
t.cc:3: ISO C++ forbids declaration of `__v' with no type
t.cc:3: parse error before `}'
[alane@wozzle /tmp]$ 

yes, i know this is *really* obscure. and it could just as well be taken as
a bug in glibc. feel free to recategorize as such if that is warranted.

problem is, it's just *so* weird, and the error message from compiling a
perfectly reasonable piece of code is totally martian. i don't know what
right fix is. i just put parens around the htonl call so it wouldn't macro
expand. fsck. [sigh]