RFE: configure time search for python

[Paul Hargrove]

We've been relying on the following in upcxx-run:
#!/usr/bin/env python

That is fine as long as there is a python in $PATH.
However, FreeBSD has chosen to approach the Python2-EOL by providing a python3 and NOT a "bare" python. I suspect there may be other (supported by us) systems where this approach is taken.

It should be possible to have configure perform a more expansive search, including for python3 as well as a --with-python=.... The she-bang line in upcxx-run can easily be rewritten at installation time. The in-build-tree uses can be (approximately) $(UPCXX_PYTHON) $(TOPSRC)/utils/upcxx-run to run the script without replacing the she-bang.

In the absence of this work, the failure mode is fairly straight-forward;

$ bin/upcxx-run -n1 -n2 ./hello.exe
env: python: No such file or directory

And the trivial fix is to append a 3 to the first line of the script.

[Paul Hargrove]

Proposed implementation of --with-python=... (in lieu of the search described here) is in pull request 163

[Paul Hargrove]

Implement and document --with-python=

This commit provides --with-python-... allowing the user running configure with the ability to override the Python interpereter to be used. Logic has been added to ensure the in-build-tree and installed upcxx-run script will both use the speficied interpreter.

Additionally, the interpreter to be used (either user-specified or default) is now subjeted to a version check in configure.

The logic in upcxx-run to perform a dynamic version check and to honor $UPCXX_PYTHON have not been removed. This mechanism does still provide the means to deal with a post-configure change to $PATH (or the python installation appearing in it).

Resolves issue #309

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