build: Fix quoting in X_AC_ZEROMQ

Wrap sources in AC_RUN_IFELSE with AC_LANG_SOURCE.

Unfortunately, expansion of AC_LANG_SOURCE seems to drop brackets
from C source, so we get char x41 instead of char x[41].
As a workaround, add an extra set of brackets around these declarations.
(Hopefully this doesn't break things elsewhere)
build: add test for alternate json-c pkg-config file

In Ubuntu, json-c pkg-config file is installed under
json-c.pc not json.pc. Test for this version as an alternate
during configure.
build: Add test for uuid library

Add test for uuid library pkg-config.
This library is necessary for build, so we
should be checking for it at configure time.
build: fix references to json.h

On ubuntu the json-c headers install into /usr/include/json-c
not /usr/include/json, so we need to remove all references to
<json/json.h> and flatten them to <json.h>.

The json-c pkgconfig file takes care of adding -I/usr/include/json
or -I/usr/include/json-c as necessary.
build: enable silent rules

All that noise in builds is hardly helpful. Enable silent automake
rules by default. This can disabled with

 make V=1

or

 ./configure --disable-silent-rules
build: docs: quiet the manpage build with silent-rules

Quiet the build of manpages when silent-rules is active.
Also, send stderr of a2x command to /dev/null by default to suppress
annoying "Note: writing..." message.