Fix incremental compilation

Author: rexim

Makefile

@@ -1,19 +1,21 @@
-build/counter: build examples/counter.c coroutine.h build/coroutine.o
+build/counter: examples/counter.c coroutine.h build/coroutine.o
 	gcc -I. -Wall -Wextra -ggdb -o build/counter examples/counter.c build/coroutine.o
 
-build/echo: build examples/echo.c3 coroutine.c3 build/coroutine.o
+.PHONY:
+examples: build/counter build/counter_cpp build/counter_c3 build/echo
+
+build/echo: examples/echo.c3 coroutine.c3 build/coroutine.o
 	cp build/coroutine.o build/echo.coroutine.o # c3c deletes the object files for some reason, so we make a copy to preserve the original
 	c3c compile -o build/echo examples/echo.c3 coroutine.c3 build/echo.coroutine.o
 
-build/counter_cpp: build examples/counter.cpp coroutine.h build/coroutine.o
+build/counter_cpp: examples/counter.cpp coroutine.h build/coroutine.o
 	g++ -I. -Wall -Wextra -ggdb -o build/counter_cpp examples/counter.cpp build/coroutine.o
 
-build/counter_c3: build examples/counter.c3 coroutine.c3 build/coroutine.o
+build/counter_c3: examples/counter.c3 coroutine.c3 build/coroutine.o
 	cp build/coroutine.o build/counter.coroutine.o # c3c deletes the object files for some reason, so we make a copy to preserve the original
 	c3c compile -o build/counter_c3 examples/counter.c3 coroutine.c3 build/counter.coroutine.o
 
-build/coroutine.o: build coroutine.c coroutine.h
+build/coroutine.o: coroutine.c coroutine.h
+	mkdir -p build
 	gcc -Wall -Wextra -ggdb -c -o build/coroutine.o coroutine.c
 
-build:
-	mkdir -p build