Append to libzmq_la_CPPFLAGS and libzmq_la_LIBADD.

If both --with-sodium and --with-pgm are defined, the latter overrides the former's CFLAGS and LIBS because libzmq_la_CPPFLAGS and libzmq_La_LIBADD are assigned to instead of appended to.

Note that we need to assign both macros to empty values (lines 231 and 233), because automake does not support += unless the variable has already been defined.