Also apply minor updates to makefile.gcc and makefile.msys

Delete the files that are unused, redundant, or fundamentally broken:
def.c, makefile.bor, makefile.knr, makefile.msc, makefile.solaris-x86,
makefile.tc3

As of July 2018, this statement is no longer necessary.

found via `codespell-q 3`
please review

Some are user facing. Some are in actual code. Most are in source comments. Also, please double check the changes in contrib/tools/pngfix.c