Implement basic generation and printing

I expect this will probably generate some impossible puzzles, but I'll
deal with that later.

Makefile

@@ -17,7 +17,7 @@
 CFLAGS += -std=c11 -pedantic -Wall -Wextra
 CFLAGS += -O2 -flto
 
-SRC = main.c
+SRC = main.c sud.c
 OBJ = $(SRC:.c=.o)
 
 sudoku: $(OBJ)
@@ -26,4 +26,7 @@ sudoku: $(OBJ)
 clean:
 	rm -f sudoku $(OBJ)
 
+main.o: sud.h
+sud.o: sud.h
+
 .PHONY: clean