build: Update scripts/makefile.* 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.