[issue17845] Clarify successful build message

[Harrison Morgan]

Harrison Morgan added the comment:

Would "external libraries" work better? It's clearly not referring to Python packages. And could be installed by a system package manager, or by yourself in a non-standard location.

Python build finished successfully!
The necessary external libraries to build these optional modules were not found:

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue17845>
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