Make USE_SYSTEM_LIBM work again

deps/Makefile

@@ -577,7 +577,12 @@ install-openlibm: $(OPENLIBM_OBJ_TARGET)
 
 ## openspecfun ##
 
-OPENSPECFUN_FLAGS = ARCH="$(ARCH)" CC="$(CC)" FC="$(FC)" AR="$(AR)" OS="$(OS)" USECLANG=$(USECLANG) USEGCC=$(USEGCC) FFLAGS="$(JFFLAGS)" USE_OPENLIBM=1
+OPENSPECFUN_FLAGS = ARCH="$(ARCH)" CC="$(CC)" FC="$(FC)" AR="$(AR)" OS="$(OS)" USECLANG=$(USECLANG) USEGCC=$(USEGCC) FFLAGS="$(JFFLAGS)"
+
+ifeq ($(USE_SYSTEM_LIBM),0)
+	OPENSPECFUN_FLAGS += USE_OPENLIBM=1
+	OPENLIBM_FAKE_TARGET = $(OPENLIBM_OBJ_TARGET)
+endif
 
 OPENSPECFUN_OBJ_TARGET = $(build_shlibdir)/libopenspecfun.$(SHLIB_EXT)
 OPENSPECFUN_OBJ_SOURCE = openspecfun/libopenspecfun.$(SHLIB_EXT)
@@ -590,7 +595,8 @@ endif
 ifeq (exists, $(shell [ -d $(JULIAHOME)/.git/modules/deps/openspecfun ] && echo exists ))
 $(OPENSPECFUN_OBJ_SOURCE): $(JULIAHOME)/.git/modules/deps/openspecfun/HEAD
 endif
-$(OPENSPECFUN_OBJ_SOURCE): openspecfun/Makefile $(OPENLIBM_OBJ_TARGET)
+
+$(OPENSPECFUN_OBJ_SOURCE): openspecfun/Makefile $(OPENLIBM_FAKE_TARGET)
 	$(MAKE) -C openspecfun $(OPENSPECFUN_FLAGS) $(MAKE_COMMON)
 	touch -c $@
 $(OPENSPECFUN_OBJ_TARGET): $(OPENSPECFUN_OBJ_SOURCE)