Added color-tests, silent-rules (Not yet implemented in Makefile.am) and
version checking to configure.ac

This version of autogen.sh attempts to dissuade people from running it when
it is not, or should not, be necessary.

Also some layout regularization and removal of introduced tab characters
(replaced with 3-character indentation).  Obsolete macros identified by
autoupdate have been removed; the replacements are all in 2.59 so
the pre-req hasn't been changed.  --enable-werror checks for support
for -Werror (or the given argument) in the compiler.  This mimics the
gcc configure option by allowing -Werror to be turned on safely; without
the option the tests written in configure itself fail compilation because
they cause compiler warnings.

separate.  The include path of 'config.h' was erroneously made relative in
pngvalid.c in libpng 1.5.7.

problems with build scripts that search for config.h and find an
inappropriate one in the standard paths.

of parallelism during 'make' the use of the same temporary file names such
as 'dfn*' can result in a race where a temporary file from one arm of the
build is deleted or overwritten in another arm.  This changes the
temporary files for suffix rules to always use $* and ensures that the
non-suffix rules use unique file names.

and rebuilt Makefile.in and configure, etc., with autoconf-2.68

instead of ANNOUNCE

adjacent @ signs with a space; this changes the concatentation
from @@@ to PNG_JOIN; that should work with all compiler
preprocessors.

pnglibconf.h.prebuilt

table, to avoid cpp failure on SunOS (introduced PNG_BUILDING_SYMBOL_TABLE
macro).

    Added a make macro DFNCPP that is a CPP that will accept the tokens in
    a .dfn file and adds configure stuff to test for such a CPP.  ./configure
    should fail if one is not available.

so it will work with automake versions earlier than 1.11.1

that CCP writes to standard output by default (Martin Banky).