added rule for markdown files without extension

Makefile

@@ -2,16 +2,19 @@
 OUT_DIR = ./output
 SRC_FILES = $(wildcard *.md)
 OUT_FILES = $(SRC_FILES:.md=.pdf)
+TARGETS   = $(SRC_FILES:.md=)
 
 PANDOC_OPT = --standalone
 
-.PHONY: all clean git test
+.PHONY: all clean test
 
-all: $(OUT_FILES)
+all: $(TARGETS)
 
 clean:
 	rm -f $(OUT_DIR)/*
 
-%.pdf: %.md
-	pandoc $(PANDOC_OPT) $^ -o $(OUT_DIR)/$@
+$(TARGETS): %: $(OUT_DIR)/%.pdf
+
+$(OUT_DIR)/%.pdf: %.md
+	pandoc $(PANDOC_OPT) $^ -o $@