Arla 0.43 autoconf warning on FreeBSD 5.5

[Garrett Wollman]

configure: WARNING: sys/libkern.h: present but cannot be compiled
configure: WARNING: sys/libkern.h:     check for missing prerequisite headers?
configure: WARNING: sys/libkern.h: see the Autoconf documentation
configure: WARNING: sys/libkern.h:     section "Present But Cannot Be Compiled"
configure: WARNING: sys/libkern.h: proceeding with the preprocessor's result
configure: WARNING: sys/libkern.h: in the future, the compiler will take precedence
configure: WARNING:     ## ------------------------------------------- ##
configure: WARNING:     ## Report this to arla-drinkers at stacken.kth.se ##
configure: WARNING:     ## ------------------------------------------- ##

A check of config.log reveals that configure erroneously tries to
include both <stdlib.h> and <sys/libkern.h>, which is doomed to
failure on FreeBSD 5.5.

A similar test of <sys/sysproto.h> also loses, because _KERNEL is not
defined, and would probably still lose if it were for the same reason
as libkern.  There probably needs to be a separate test macro for "see
if we can compile this in the kernel environment".

-GAWollman