Allow multiple arguments.

Fixes various static library linker failures ("bad extended name entry
at header"), because the wrong "ranlib" was used.

Reduce dependencies on global variables.

Got lost in the previous commit.

Not used anymore.

zlib cannot be built out-of-tree, and previously, we were building in
the "src" directory, not in the "build" directory.  The
make_build_path() return value was unused.

Fixes a bug that occurs when runing "build.py" from inside the
"win32" directory.

Fixes in-tree build when the script is called as "win32/build.py"
and not "./win32/build.py".

Allow multiple targets to share a library directory tree.