Remove duplicate poller decision making

The decision about the poller mechanism to use (select, poll, ...)
was done twice: once by the build system and once by the code in
poller.hpp. As the build-system can actually detect the mechanisms
available, prefer that result to the hard coded defaults in
poller.hpp.

At the same time, remove the duplicate detection of select() vs.
poll()-variant from proxy.cpp, signaler.cpp and zmq.cpp.

This patch has not been tested on many build platforms: especially
the cmake build needs testing / patching. For the other builds,
hard code the result as these these are all Windows platforms.

builds/mingw32/Makefile.mingw32

@@ -1,5 +1,5 @@
 CC=gcc
-CFLAGS=-Wall -Os -g -DDLL_EXPORT -DFD_SETSIZE=1024 -I.
+CFLAGS=-Wall -Os -g -DDLL_EXPORT -DFD_SETSIZE=1024 -DZMQ_USE_SELECT -I.
 LIBS=-lws2_32
 
 OBJS = ctx.o reaper.o dist.o err.o \