compiler.pri: allow version-less Boost to be used on Windows.
compiler.pri: allow version-less Boost to be used on Windows.

This allows Mumble to build with Boost that has been
configured with the --layout=tagged or --layout=system
options.

The win32-static build environment uses this kind of
Boost install.

This means we'll be able to keep Boost up-to-date without
modifying compiler.pri each time.

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