I wonder if it is a race condition in the Makefile. libmux.so is built from code in the package. It isn't an external dependency. From the Makefile:
MUX_LIBS = -lmux
...
all: libmux.so netmux slave stubslave links subdirs
...
stubslave: stubslave.o
$(CXX) $(ALLCXXFLAGS) -o stubslave stubslave.o -L. $(LIBS) $(MUX_LIBS) $(STUBLIBS)
I bet that should be
stubslave: stubslave.o libmux.so
$(CXX) $(ALLCXXFLAGS) -o stubslave stubslave.o -L. $(LIBS) $(MUX_LIBS) $(STUBLIBS)
This has probably been an issue forever, but no one has tried to build it on enough cores, yet. And, likewise, I won't know for certain that it is fixed without someone without enough cores.
Stephen