Apply the following updates:
* Tidy up the compiler flag definitions.
* Update the Darwin, Linux and MSYS makefiles to match the compiler
flags used in scripts/makefile.clang and scripts/makefile.gcc.
* Add the `pngtest-static` target in the Darwin makefile, following
on the Linux makefile.
* Rewrite some of the implicit make rules to match one another more
consistently.
* Make corrections in the copyright years to match git log.
In most of the legacy makefiles (with only a few notable exceptions
such as makefile.linux), the "install" targets have been broken since
libpng-1.5.0beta01. Specifically, the "install-shared" targets produced
incorrect library names, due to a missing symbol in the definition of
the LIBSOREL macro.
Rather than correcting this long-standing and long-untested error, we
decided to remove the support for "make install" altogether, from all
legacy makefiles. Further use of "make install", "make install-static"
or "make install-shared" will fail with an error message.
Going forward, the only tested and supported manners to install libpng
shall be the conventional build and install procedures driven by the
configure script or by the CMake file.
Considering that "-Wextra" is a more descriptive alternative to "-W",
and that "-Wundef" is a highly useful warning option that has been
available in ancient versions of gcc (version 2.x), we replace all
occurrences of "-W -Wall" with "-Wall -Wextra -Wundef".
Also clean up the makefiles.
Delete variables (ALIGN, ARCH, DOCS) and targets (writelock) that are
no longer necessary.
Reorder the object file lists alphabetically, consistently across all
makefiles and build scripts.
Apply other minor fixes.
