Problem: Cumbersome to compile z* files in labs folder

Solution: Changed the autotools configuration

Prerequisites: libczmq is installed

Step 1: ./configure --enable-labs=yes
Step 2: make
Step 3: make install

Now you've got a library libczmq_labs installed on your system which
can be used in third party projects.

Known issues: Couln't get czmq_labs_selftest to compile, thus those 
parts are commented in the labs/Makefile.am

* Let's try this again, thanks to git cherry-pick :)