Bug in Regular Expression in Makefile (#4682)

Summary:
False-negative about path not existing. The regex is ignoring the "." in front of a path.
Example: "./path/to/file"
Pull Request resolved: https://github.com/facebook/rocksdb/pull/4682

Differential Revision: D13777110

Pulled By: sagar0

fbshipit-source-id: 9f8173b7581407555fdc055580732aeab37d4ade

Makefile

@@ -198,7 +198,7 @@ include make_config.mk
 CLEAN_FILES += make_config.mk
 
 missing_make_config_paths := $(shell				\
-	grep "\/\S*" -o $(CURDIR)/make_config.mk | 		\
+	grep "./\S*\|/\S*" -o $(CURDIR)/make_config.mk | 	\
 	while read path;					\
 		do [ -e $$path ] || echo $$path; 		\
 	done | sort | uniq)