feat: CLI-only version (without GTK)

Modify the build system and the source
files to support building a CLI-only
version with only glib2 as a dependency.
Use CLI_ONLY=1 ./configure, then make clean all.

Makefile

@@ -4,7 +4,9 @@
 
 all:
 	@echo
-	@echo "Please run \`bash configure' first."
+	@echo "Please run \`./configure' first."
+	@echo "To build a CLI-only version,"
+	@echo "run \`CLI_ONLY=1 ./configure' instead."
 	@echo "Note that dvdisaster requires GNU make to build."
 	@echo "Under non-Linux systems, it might be known as \`gmake'."
 	@echo