build:  added PREFIX to samples/Makefile (don't know how the linux build worked without this!)

git-svn-id: svn://dev.exiv2.org/svn@3092 b7c8b350-86e7-0310-a4b4-de8f6a8f16a3
Update to 3091

git-svn-id: svn://dev.exiv2.org/svn@3093 b7c8b350-86e7-0310-a4b4-de8f6a8f16a3