Make libcoap examples build

Running configure --prefix=/tmp/libcoap --enable-shared on Ubuntu and on Mac fails.

We get one warning about missing doxgen and an error for the missing a2x tool, see also #20 (closed).

Add support in prereq.sh at the top level to install / add the missing toolchain or decide that we do not need to ship the libcoap examples.

Note that due to our upcoming modifications to libcoap, the examples might not work anyway without major adjustments in the code.