Support libzmq's Autotools Install: pkg-config

[ax3l]

Following #127 and zeromq/libzmq#2621 (comment), the libzmq install via autotools is still a thing and used in packaging systems, too.

Albeit the autotools install lacks the install of CMake config modules, it would be great if we could add support for libzmq autotools-installs anyway.

One way could be to support the always-installed pkg-config module named libzmq in cppzmq's CMakeLists.txt.

For autotools based installs we could as a fallback to the current logic piggyback on pkg-config. This could look like this:

find_package(PkgConfig)
pkg_check_modules(PC_LIBZMQ QUIET libzmq)

set(ZeroMQ_VERSION ${PC_LIBZMQ_VERSION})
find_library(ZeroMQ_LIBRARY NAMES libzmq.so
             PATHS ${PC_LIBZMQ_LIBDIR} ${PC_LIBZMQ_LIBRARY_DIRS})
find_library(ZeroMQ_STATIC_LIBRARY NAMES libzmq.a
             PATHS ${PC_LIBZMQ_LIBDIR} ${PC_LIBZMQ_LIBRARY_DIRS})

[bluca]

Sounds good, could you please send a PR please?

Also to ensure maximum backward compatibility I would suggest looking for the CMake files first, and then pkgconfig as a fallback.

Otherwise a third project might start picking the system library unintentionally, for example.

[ax3l]

sure, can do. the only point is that this is likely only helpful for *nix systems (and I have no access to other envs such as OSX/Win)

[bluca]

That's OK - if it's a fallback, it won't hurt. Thanks!

[ax3l]

awww, since the last stable release someone started patching in targets for the two libzmq libs... The patch I proposed above won't cut that properly I guess... #118