need hint about build environment screwup

[Gary Aitken]

In trying to build
  /usr/ports/sysutils/lsof 
I get the following:

In file included from /usr/include/_ctype.h:94,
                 from /usr/include/ctype.h:46,
                 from lsof.h:49,
                 from dmnt.c:39:
/usr/include/runetype.h:92: error: expected '=', ',', ';', 'asm' or '__attribute__' before 'const'
/usr/include/runetype.h: In function '__getCurrentRuneLocale':
/usr/include/runetype.h:96: error: '_ThreadRuneLocale' undeclared (first use in this function)
/usr/include/runetype.h:96: error: (Each undeclared identifier is reported only once
/usr/include/runetype.h:96: error: for each function it appears in.)

I'm running 9.1 RELEASE on an amd64 with a recently updated ports tree, 
and recently upgraded to using pkgng
/etc/make.conf contains
  WITH_PKGNG=yes

I suspect something is wrong with the build environment, but haven't a clue.
I'm trying to avoid rebuilding everything, and would like to find out what's
screwed up.
Since lsof basically doesn't depend on anything else, I'm baffled as to 
what got messed up.

Can someone give me a hint?

Thanks,

Gary