MINOR: cpuset: define a platform-independent cpuset type

This module can be used to manipulate a cpu sets in a platform agnostic
way. Use the type cpu_set_t/cpuset_t if available on the platform, or
fallback to unsigned long, which limits de facto the maximum cpu index
to LONGBITS.
diff --git a/Makefile b/Makefile
index 3c9109b..46ae734 100644
--- a/Makefile
+++ b/Makefile
@@ -563,6 +563,10 @@
 OPTIONS_LDFLAGS += -Wl,$(if $(EXPORT_SYMBOL),$(EXPORT_SYMBOL),--export-dynamic)
 endif
 
+ifneq ($(USE_CPU_AFFINITY),)
+OPTIONS_OBJS   += src/cpuset.o
+endif
+
 ifneq ($(USE_OPENSSL),)
 SSL_INC =
 SSL_LIB =