How to link compiler to mvn for JRuby Windows builds?

How do I link MinGW’s C++ compiler to the build environment for JRuby on

Attempting to install 1.7.11 on Windows an mvn -Pbootstrap fails with:

[WARNING] ERROR: Error installing
[WARNING] ERROR: Failed to build gem native extension.

Which is presumably because mvn isn’t linked in to the MinGW C++
compiler on Windows. What is the correct way to set up the development
environment so mvn can use the C++ compiler?

I have a MinGW installed and I can use it to go into directories and
issue make commands successfully, but only from the msys.bat shell of
MinGW and not from mvn.