Build failure on Debian unstable

[Sven Joachim]

Please have a look at http://bugs.debian.org/423462, it says that
Emacs currently fails to build:

[Quoting from the bug report]

,----
| In file included from ./config.h:979,
|                  from dispnew.c:23:
| ./s/gnu-linux.h:49:24: error: operator '>' has no left operand
| ./s/gnu-linux.h:52:24: error: operator '>=' has no left operand
| ./s/gnu-linux.h:55:24: error: operator '>=' has no left operand
| 
| The offending code in gnu-linux.h references LINUX_VERSION_CODE,
| 
| #if LINUX_VERSION_CODE > 0x10200
| #define LINUX_SIGIO_DOES_WORK
| #endif /* LINUX_VERSION_CODE > 0x10200 */
| #if LINUX_VERSION_CODE >= 0x20000
| #define LINUX_MAP_SHARED_DOES_WORK
| #endif /* LINUX_VERSION_CODE >= 0x20000 */
| #if LINUX_VERSION_CODE >= 0x20400
| 
| ... which is defined in /usr/include/linux/version.h:
| 
| #define LINUX_VERSION_CODE
| #define KERNEL_VERSION(a,b,c) (((a) << 16) + ((b) << 8) + (c))
`----

I verified that this does indeed happen.  While it is an incompatible
change that the Debian maintainers did not give LINUX_VERSION_CODE a
version, I tend to agree Steve Langasek that it is very unreliable to
check LINUX_VERSION_CODE to determine the version of the Linux kernel,
since the kernel version is not really related to what
/usr/include/linux/version.h states about it (and it's entirely
possible to build Emacs with a new kernel but run it under an older
one).