One last fix to Makefile

Summary: Based on comment from D37455

Test Plan: make install after make static_lib

Reviewers: meyering

Subscribers: dhruba

Differential Revision: https://reviews.facebook.net/D37461

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 || :
 
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