Makefile: automatically generate dependencies.

This uses a trick from scottmcpeak.com/autodepend/autodepend.html. It
may seem like overkill for such a small codebase, but note that header
modifications did not trigger rebuilds in the affected object files in
the existing Makefile.

.gitignore

@@ -1,2 +1,3 @@
 *.o
+*.d
 ssd1306_bin