Only builds with --enable-experimental require zlib currently.
A base install of mingw will not include the development headers and
library. libwebp itself will now build in such environments.
Additionally, remove -lz from **/Makefile.am, -lz will be added to LIBS
by AC_CHECK_LIB when necessary.
Change-Id: Iae8319cdf00162ecb7ed44661c02f40beb34f155