configure: Add an option to enable all optional features/plugins

This has probably no real practical use, but it simplifies testing.

m4/macros/enable-disable.m4

@@ -2,6 +2,7 @@
 # ARG_ENABL_SET(option, help)
 # ---------------------------
 # Create a --enable-$1 option with helptext, set a variable $1 to true/false
+# All $1 are collected in the variable $disabled_by_default
 AC_DEFUN([ARG_ENABL_SET],
 	[AC_ARG_ENABLE(
 		[$1],
@@ -14,7 +15,8 @@ AC_DEFUN([ARG_ENABL_SET],
 		fi],
 		[patsubst([$1], [-], [_])=false
 		patsubst([$1], [-], [_])_given=false]
-	)]
+	)
+	disabled_by_default=${disabled_by_default}" patsubst([$1], [-], [_])"]
 )
 
 # ARG_DISBL_SET(option, help)