Overhauled building

- config.h -> config.def.h
- Create config.h during make, if it does not exist
- Nice make output
- Use XFLAGS and XLIBS to include additional compile-time features,
  which depend on third-party libraries

.gitignore

@@ -1,5 +1,6 @@
+config.h
 *.o
 sxiv
 cscope.out
-release
 tags
+release