Remove spurious #ifdef PNG_sRGB_SUPPORTED
Spurious because it is within PNG_sRGB_READ_SUPPORTED

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