[libpng15] Fixed CMakelists.txt to allow building a single variant of the

    library (Claudio Bley):
  Introduced a PNG_LIB_TARGETS variable that lists all activated library
    targets.  It is an error if this variable ends up empty, ie. you have
    to build at least one library variant.
  Made the *_COPY targets only depend on library targets actually being build.
  Use PNG_LIB_TARGETS to unify a code path.
[libpng15] Changed the CREATE_SYMLINK macro to expect the full path to a file

as the first argument. When symlinking the filename component of that path is
determined and used as the link target.
[libpng15] Use copy_if_different in the CREATE_SYMLINK macro.
[libpng15] Eliminated two warnings from the Intel C compiler.
[libpng15] Imported from libpng-1.5.15beta09.tar
[libpng15] Bump version to 1.5.15beta10
[libpng15] Imported from libpng-1.5.15rc01.tar