freebsd stable - Sep 2016 - Source upgrade to 10.3: Undefined symbol "__set_error_selector"

FYI (issue is resolved so I'm just reporting for posterity)...

I have four offline ("air gapped") FreeBSD systems with nearly
identical
hardware.  Two started life as 10.1-RELEASE and the other two started 
life as 10.2-RELEASE.

All are kept up to date by bringing over /usr/src for their associated 
releases (using 'svn co https://svn.freebsd.org/base/releng/10.x') and 
make/buildworld.

All were upgraded to 10.3-p5 and failed 'make installworld' with:

/lib/libthr.so.3: Undefined symbol "__set_error_selector"

All resolved with 'cd /usr/src/lib/libc && make install &&
cd /usr/src
&& make installworld'

Chris