Problem: PKG_CHECK_MODULES doesn't add to LIBS on android

Solution: Add -lzmq to LIBS manually

Took me quite a while to figure out why czmq was failing at runtime on android, but it's because it wasn't actually getting linked to libzmq.  Looking at `config.log`, I saw that `configure` was not adding it to `LIBS`, even though libzmq was getting successfully detected.  This works fine on my desktop, and I'm not sure why it's different, but `PKG_CHECK_MODULES` doesn't seem to be adding to `LIBS` when compiling for android.

It seems like `PKG_CHECK_MODULES` was added in a recent commit (https://github.com/zeromq/czmq/commit/313714f8b73c23d68b1216bc2df3d0ccc6ab1ffd).  I'm no expert on C build systems, but this seemed like a reasonable way to go about a workaround.