Fix make install when there is no shared lib

Summary: make install fails when there is no shared lib. We need to revert the conditions, which will have the same effect, but without the failure

Test Plan: make install after only compiling static library

Reviewers: meyering

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D37455

Makefile

@@ -836,7 +836,7 @@ install-shared: install-headers $(SHARED4)
 
 # install static by default + install shared if it exists
 install: install-static
-	[ -e $(SHARED4) ] && $(MAKE) install-shared
+	[ ! -e $(SHARED4) ] || $(MAKE) install-shared
 
 #-------------------------------------------------