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This was part of the original design of the module
API (https://lists.gnu.org/archive/html/emacs-devel/2015-02/msg00960.html),
but I didn't take it into account when adding the should_quit
function.

Instead of duplicating the environment fields or using the C
preprocessor, use configure to build emacs-module.h.

* configure.ac: Expand emacs-module.h template.