Set a xy_given variable for a --enable/disable-xy option

This additional variable allows a check if an option was
explicitly given or implicitly set using the default.

m4/macros/enable-disable.m4

@@ -6,12 +6,14 @@ AC_DEFUN([ARG_ENABL_SET],
 	[AC_ARG_ENABLE(
 		[$1],
 		AS_HELP_STRING([--enable-$1], [$2]),
-		[if test x$enableval = xyes; then
+		[patsubst([$1], [-], [_])_given=true
+		if test x$enableval = xyes; then
 			patsubst([$1], [-], [_])=true
 		 else
 			patsubst([$1], [-], [_])=false
 		fi],
-		patsubst([$1], [-], [_])=false
+		[patsubst([$1], [-], [_])=false
+		patsubst([$1], [-], [_])_given=false]
 	)]
 )
 
@@ -22,11 +24,13 @@ AC_DEFUN([ARG_DISBL_SET],
 	[AC_ARG_ENABLE(
 		[$1],
 		AS_HELP_STRING([--disable-$1], [$2]),
-		[if test x$enableval = xyes; then
+		[patsubst([$1], [-], [_])_given=true
+		if test x$enableval = xyes; then
 			patsubst([$1], [-], [_])=true
 		 else
 			patsubst([$1], [-], [_])=false
 		fi],
-		patsubst([$1], [-], [_])=true
+		[patsubst([$1], [-], [_])=true
+		patsubst([$1], [-], [_])_given=false]
 	)]
 )