[libpng17] Made changes for compatibility with automake 1.14:

    1) Added the 'compile' program to the list of programs that must be cleaned
       in autogen.sh
    2) Added 'subdir-objects' which causes .c files in sub-directories to be
       compiled such that the corresponding .o files are also in the
       sub-directory.  This is because automake 1.14 warns that the
       current behavior of compiling to the top level directory may be removed
       in the future.
    3) Updated dependencies on pnglibconf.h to match the new .o locations and
       added all the files in contrib/libtests and contrib/tools that depend
       on pnglibconf.h
    4) Added 'BUILD_SOURCES = pnglibconf.h'; this is the automake recommended
       way of handling the dependencies of sources that are machine generated;
       unfortunately it only works if the user does 'make all' or 'make check',
       so the dependencies (3) are still required.
[libpng17] Cleaned up (char*) casts of zlib messages. The latest version

of the Intel C compiler complains about casting a string literal as (char*),
so copied the treatment of z_const from the library code into pngfix.c
[libpng17] Simplified error message code in pngunknown. The simplification has

the useful side effect of avoiding a bogus warning generated by the latest
version of the Intel C compiler (it objects to
condition ? string-literal : string-literal).
[libpng17] Make autogen.sh work with automake 1.13 as well as 1.14. Do this

by always removing the 1.14 'compile' script but never checking for it.