Libzmq uses C++ standard library features, so users of it should link
against that as well when statically linking.
Add it to Libs.private so users using pkg-config automatically gets the
correct linker flags.
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
The ${libdir} was getting replaced/removed by configure_file() making pkg-config give bad flags: -L -lzmq
My fix was to add @ONLY to configure_file() so ${} style pkg-config substitutions are left alone.
In addition, I put the other typical ${} substitutions back into the libzmq.pc, since its now safe.
