Fix dylib install_name on OS X.

This resulted in PREFIX/PREFIX/libwebvfx.1.dylib instead of
PREFIX/libwebvfx.1.dylib as intended.
Use default texture type to fix building for Windows.

Non-power-of-two 2D textures are supported by OpenGL 2.0 or later, which
is old enough to require in 2013. See QGLFramebufferObject docs.
Merge pull request #5 from ddennedy/master

Build improvements for OS X and Windows