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.
Merge pull request #781 from pmienk/master

Simply configure.ac to prevent contradictory discovery of libzmq.
Merge pull request #782 from jemc/master

Problem: Android needs to provide special LDFLAGS to libtool linking
Problem: misuse of include files in zsys

All system specific include files go into czmq_prelude.h. zsys was using
errno.h for no good reason.

Solution: just remove this, errno is already included in czmq_prelude.h.

Secondary issues fixed on the way:

- zsys was sending its UDP sendto() failures to stderr, I changed this
  to send to zsys_error() as for all other error reports.

- zarmour was including zarmour.h which is redundant -- removed.
Problem: zbeacon contains bogus comments

These were left over from the original work, serve no purpose.

Solution: remove the junk comments.
Problem: zsocket tried to unbind wildcard endpoint

This was not documented and recently changed in ZeroMQ, so now causes
an assertion failure.

Solution: do not use '*' interface in tests, use 127.0.0.1 instead.
Merge pull request #783 from hintjens/master

Problem: misuse of include files in zsys
Update CONTRIBUTING.md
Problem: PKG_CHECK_MODULES doesn't add to LIBS on android
Solution: Add -lzmq to LIBS manually