cygwin - locale_utils.cc:112:3: error: ‘unsetenv’ was not declared in this scope

Today's git fails with the following compile error on Windows 10.

locale_utils.cc:112:3: error: ‘unsetenv’ was not declared in this scope
   unsetenv( "LANG" );

First few lines of config.log

This file contains any messages produced by compilers while
running configure, to aid debugging if configure makes a mistake.

It was created by mosh configure 1.3.2, which was
generated by GNU Autoconf 2.69.  Invocation command line was

  $ ./configure

## --------- ##
## Platform. ##
## --------- ##

hostname = exeter
uname -m = x86_64
uname -r = 3.0.1(0.338/5/3)
uname -s = CYGWIN_NT-10.0
uname -v = 2019-02-20 10:19

/usr/bin/uname -p = unknown
/bin/uname -X     = unknown

/bin/arch              = x86_64
/usr/bin/arch -k       = unknown
/usr/convex/getsysinfo = unknown
/usr/bin/hostinfo      = unknown
/bin/machine           = unknown
/usr/bin/oslevel       = unknown
/bin/universe          = unknown

PATH: /usr/local/bin
PATH: /usr/bin
PATH: /cygdrive/c/ProgramData/DockerDesktop/version-bin

The previous version I had was from Jan 2018, and it compiled OK for me.

Start by inspecting locale_utils.cc around line 112 and the Cygwin platform details in config.log. Compare the environment-unsetting path with the platform conditions shown there. Done means mosh configures and compiles on Windows 10/Cygwin without the unsetenv declaration error.