If a zlib source tree exists besides the libpng source tree in the
same parent directory, the pngminus build may fail, unless the zlib
tree is built fully. In order to avoid this failing scenario, do not
use the custom-built zlib by default.

(The custom-built zlib is still necessary on platforms that lack a
system-built zlib.)

Change the license to MIT.
Move the license text from the source files to a LICENSE file.
Move the change log from the source files to a CHANGES file.
Delete the Turbo C makefile and simplify the Linux makefile heavily.
Create explicitly named static and shared executables in the makefile.
Refresh the README file a bit from the twenty year old one.
Signed-off-by: NWillem van Schaik <willem@schaik.com>
Signed-off-by: NCosmin Truta <ctruta@gmail.com>