[gn build] Put HAVE_LIBZ and HAVE_ZLIB_H behind llvm_enable_zlib too.

In the CMake build, the HAVE_ vars are set based on system inspection,
and LLVM_ENABLE_ZLIB is set to false if neither's found. The GN build
doesn't do autodetection like this.

With this change, people can set llvm_enable_zlib=true on Windows
and as long as they provide a zlib.lib things should actually work.

(https://reviews.llvm.org/D79219 will remove 2 of the 3 config.h
values, hopefully soon. This change here just makes things a tiny
bit easier until that change is in.)