build: Move scripts/*.m4 to scripts/autoconf/

Move all Autoconf macro files (except for those that need to be in
the top-level directory) to their own subdirectory scripts/autoconf/

In this commit, we introduce a better way to organize the scripts dir,
and we make a better separation between the build scripts under the
libpng license vs. the build scripts that fall under other licenses.

Please see scripts/autoconf/README.md for more information.
11 files changed