Python27 install problem - Undefined symbol rl_callback_sigcleanup

[Per olof Ljungmark]

This is a recent 11-STABLE jail but it may contain cruft from a previous 
10 (or maybe even earlier) install. Portstree updated today.

I've done the mergemaster bit and readline-8.0.0 is present, 
rl_callback_sigcleanup eists in /usr/local/include/readline/readline.h.

Some relevant parts of the build:
...
*** WARNING: renaming "readline" since importing it failed: 
build/lib.freebsd-11.2-STABLE-amd64-2.7/readline.so: Undefined symbol 
"rl_callback_sigcleanup"
...
Failed to build these modules:
readline
...
make install
===>  Installing for python27-2.7.16_1
===>  Checking if python27 is already installed
actual-package-depends: dependency on /lib/libreadline.so.8 not 
registered (normal if it belongs to base)
===>   Registering installation for python27-2.7.16_1
pkg-static: Unable to access file 
/usr/ports/lang/python27/work/stage/usr/local/lib/python2.7/lib-dynload/readline.so:No 
such file or directory
*** Error code 74
...