automake 1.14 updates Changes for compatibility with automake 1.14: 1) Add the 'compile' program to the list of programs that must be cleaned in autogen.sh 2) Add '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) Update dependencies on pnglibconf.h to match the new .o locations and add all the files in contrib/libtests and contrib/tools that depend on pnglibconf.h 4) Add '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.

Signed-off-by: John Bowler <jbowler@acm.org>
Clean up (char*) casts of zlib messages Because the latest version of the Intel C compiler complains about casting a string literal as (char*), so copy the treatment of z_const from the library code into pngfix.c

Signed-off-by: John Bowler <jbowler@acm.org>
Simplify 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).

Signed-off-by: John Bowler <jbowler@acm.org>