Problem: Android ndk #defines errno as a preprocessor macro

Solution: Guard errno declaration in zsys
Android macro defined here: https://android.googlesource.com/platform/prebuilts/ndk/+/master/9/platforms/android-19/arch-arm/usr/include/errno.h
Merge pull request #780 from jemc/errno_guard

Problem: Android ndk #defines errno as a preprocessor macro
Simply configure.ac to prevent contradictory discovery of libzmq.