[js] Improve default for JSENGINE

Previously the default was always "node", even if nodejs wasn't
detected by configure.  This leads to a confusing failure from
"make check" if you have another support JS engine installed
but not node.

Now the default it picked based on which engines configure
found.  If only one was detected, that should be used.  If
multiple are, you can override the default choice by specifying
e.g. ENGINE=jsc on the make command line.

Fixes #2453

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