split out recipe with two targets to avoid gmake deprecation (#4317)

Author: btovar

rules.mk

@@ -22,7 +22,10 @@ else
 	$(CCTOOLS_LD) -o $@ $(CCTOOLS_INTERNAL_LDFLAGS) $(LOCAL_LDFLAGS) $^ $(LOCAL_LINKAGE) $(CCTOOLS_EXTERNAL_LINKAGE)
 endif
 
-%.so %.dylib:
+%.so:
+	$(CCTOOLS_LD) -o $@ -fPIC $(CCTOOLS_DYNAMIC_FLAG) $(CCTOOLS_INTERNAL_LDFLAGS) $(LOCAL_LDFLAGS) $^ $(LOCAL_LINKAGE) $(CCTOOLS_EXTERNAL_LINKAGE)
+
+%.dylib:
 	$(CCTOOLS_LD) -o $@ -fPIC $(CCTOOLS_DYNAMIC_FLAG) $(CCTOOLS_INTERNAL_LDFLAGS) $(LOCAL_LDFLAGS) $^ $(LOCAL_LINKAGE) $(CCTOOLS_EXTERNAL_LINKAGE)
 
 # Cancel Make defined implicit rule: