qmake: Add QMAKE_SONAME_PREFIX variable

If defined, the value of this variable is prepended to the built shared
library's SONAME identifier.

For more information, see: qmake/doc/src/qmake-manual.qdoc#qmake-soname-prefix

Task-number: QTBUG-31814
Change-Id: I4bceaf0c93162e4fad6bb424af1b82e74d38acdc
Reviewed-by: Jake Petroules <jake.petroules@petroules.com>