Updated "make clean" to remove all .o files

Summary:
The old Makefile did not remove ALL .o and .d files, but rather only
those that happened to be in the root folder and one-level deep. This was causing
issues when recompiling files in deeper folders. This fix now causes make clean
to find ALL .o and .d files via a unix "find" command, and then remove them.

Test Plan:
make clean;
make all -j 32;

Reviewers: haobo, jpaton, dhruba

Reviewed By: haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D11493

Makefile

@@ -140,8 +140,9 @@ valgrind_check: all $(PROGRAMS) $(TESTS)
 	done
 
 clean:
-	-rm -f $(PROGRAMS) $(BENCHMARKS) $(LIBRARY) $(SHARED) $(MEMENVLIBRARY) $(THRIFTSERVER) */*.o */*/*.o ios-x86/*/*.o ios-arm/*/*.o build_config.mk */*.d */*/*.d
+	-rm -f $(PROGRAMS) $(BENCHMARKS) $(LIBRARY) $(SHARED) $(MEMENVLIBRARY) $(THRIFTSERVER) build_config.mk
 	-rm -rf ios-x86/* ios-arm/*
+	-for x in `find . -name "*.[od]"`; do rm $$x; done
 
 $(LIBRARY): $(LIBOBJECTS)
 	rm -f $@