Generate ctags when building if DEBUG environment variable is set.
Don't include deprecated librsvg header.
Fix some errors when compiling with clang.

clang doesn't seem to link unused linker flags, so only use them when
linking the final binary, not when compiling intermediate object files.
Remove uses of deprecated function "gtk_tool_button_new_from_stock"
Merge branch 'master' of https://github.com/orodley/chessboard

Conflicts:
	main.c
	src/board_display.c