Kind of sort of fix xptime build (needs math lib now)

There's some GNU make sorcery still needed here to address build errors
(though it does generate a working executable):
/usr/bin/ld: cannot find gcc.linux.x64.obj.debug: file format not recognized
/usr/bin/ld: cannot find gcc.linux.x64.lib.debug: file format not recognized
/usr/bin/ld: cannot find gcc.linux.x64.exe.debug: file format not recognized

src/xpdev/GNUmakefile

@@ -35,7 +35,7 @@ $(WRAPTEST): $(MTOBJODIR)/wraptest.o $(DEPS) | $(EXEODIR)
 
 $(XPTIME): $(OBJODIR)/xptime.o $(XPDEV_LIB_BUILD)
 	@echo Linking $@
-	$(QUIET)$(CC) -o $@ $(LDFLAGS) $^ $(XPDEV_LIB_BUILD)
+	$(QUIET)$(CC) -o $@ $(LDFLAGS) $^ $(XPDEV_LIB_BUILD) -lm
 
 $(XPDEV_LIB_BUILD): $(OBJS) | $(OBJODIR)
 	@echo Creating $@