Override default installation directory on Windows only

As currently implemented, the `src/CMakeLists.txt` file overrides the system default installation path on all operating systems. Checking `CMAKE_INSTALL_PREFIX_INITIALIZED_TO_DEFAULT` has the (unintentional?) consequence of prohibiting installations to the system default directories on my machine even if the user specifies `CMAKE_INSTALL_PREFIX` when cmake is invoked for the first time.

The proposed patch modifies the default installation directory only on Windows systems. This appears to be the desired behavior given the text in the nearby comment.