Fix dylib install_name on OS X.
Fix dylib install_name on OS X.

This resulted in PREFIX/PREFIX/libwebvfx.1.dylib instead of
PREFIX/libwebvfx.1.dylib as intended.

rectalogic/webvfx,rectalogic/webvfx,mltframework/webvfx,mltframework/webvfx,rectalogic/webvfx,mltframework/webvfx,rectalogic/webvfx