Add the MP4V2_ prefix to CMake options

[corporateshark]

When this library is a part of another CMake-based project, global options BUILD_SHARED and BUILD_UTILS interfere with other projects.

Renamed both CMake options as follows:

MP4V2_BUILD_SHARED
MP4V2_BUILD_UTILS

[ferdnyc]

This will also causes the cmake-gui interface to group the options together under an MP4V2 heading. Right now they're grouped under "BUILD" which doesn't make sense (particularly when used as a subproject).

Before:

After:

[ferdnyc]

(Although, honestly, BUILD_SHARED should probably be replaced with support for the built-in CMake variable BUILD_SHARED_LIBS.)