Problem: pkg-config file cannot be used for static linking on CentOS 7

Solution: add -lm to Libs.private of libzmq.pc so that the std::ceil
usage in src/decoder_allocators.cpp is satisfied during static linking
on CentOS 7.

See https://github.com/zeromq/libzmq/issues/3710 for a reproducer.

src/libzmq.pc.in

@@ -7,6 +7,6 @@ Name: libzmq
 Description: 0MQ c++ library
 Version: @VERSION@
 Libs: -L${libdir} -lzmq
-Libs.private: -lstdc++ @pkg_config_libs_private@
+Libs.private: -lstdc++ -lm @pkg_config_libs_private@
 Requires.private: @pkg_config_names_private@
 Cflags: -I${includedir} @pkg_config_defines@