add c example to makefile and fix "make clean"

examples/Makefile

@@ -2,7 +2,7 @@ include ../build_config.mk
 
 .PHONY: clean
 
-all: simple_example column_families_example compact_files_example
+all: simple_example column_families_example compact_files_example simple_example-c
 
 simple_example: simple_example.cc
 	$(CXX) $(CXXFLAGS) $@.cc -o$@ ../librocksdb.a -I../include -O2 -std=c++11 $(PLATFORM_LDFLAGS) $(PLATFORM_CXXFLAGS) $(EXEC_LDFLAGS)
@@ -13,5 +13,8 @@ column_families_example: column_families_example.cc
 compact_files_example: compact_files_example.cc
 	$(CXX) $(CXXFLAGS) $@.cc -o$@ ../librocksdb.a -I../include -O2 -std=c++11 $(PLATFORM_LDFLAGS) $(PLATFORM_CXXFLAGS) $(EXEC_LDFLAGS)
 
-clean: simple_example column_families_example compact_files_example
-	rm -rf ./simple_example ./column_families_example ./compact_files_example
+simple_example-c:
+	$(CXX) $(CXXFLAGS) -xc $@.c -o$@ ../librocksdb.a -I../include -O2 $(PLATFORM_LDFLAGS) $(PLATFORM_CXXFLAGS) $(EXEC_LDFLAGS)
+
+clean:
+	rm -rf ./simple_example ./column_families_example ./compact_files_example ./simple_example-c