The msg_limits.c file has this chunk of code in it:

#if (defined(__FreeBSD__) && (__FreeBSD_version < 410000)) || (defined __bsdi__)
#define KERNEL 1
#else
#define _KERNEL 1
#endif

Under NetBSD this conditional obviously falls to the #else part, which defines the _KERNEL macro.  This causes later problems during the inclusion of sys/ipc.h and sys/msg.h because they try to pull in many stuff that is not available in userland -- as the _KERNEL macro name implies, defining it means that you are building the kernel!

As an example, see what the compiler emits based on a simple test case that defines _KERNEL and later includes sys/msg.h:

In file included from test.c:2:
/usr/include/sys/msg.h:193:23: sys/systm.h: No such file or directory
In file included from test.c:2:
/usr/include/sys/msg.h:200: error: parse error before "copyin_t"
/usr/include/sys/msg.h:202: error: parse error before "copyout_t"

One possible fix could be to change the #else line with '#elif !defined(__NetBSD__)', but that is ugly and extremely delicate (e.g. OpenBSD probably has the same problem, but I do not know).  It does not fix the real problem in a clean way.

The best solution to address this problem is to add a check in the configure script that detects if we need to define anything (be it KERNEL or _KERNEL) to get the definition of 'struct msginfo' regarless of the BSD flavour/version.  This can be achieved by using the AC_COMPILE_IFELSE macro.

The attached patch implements the latter solution.