./configure: cleanup '--enable-background' option description

Before the patch ./configure --help shown:
  --enable-background     Support for background file operations [[yes]]

After the patch:
  --enable-background     Support for background file operations [yes]

Changed whitespace: converted tabs to spaces.

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>

m4.include/mc-background.m4

@@ -5,15 +5,15 @@ dnl
 AC_DEFUN([MC_BACKGROUND],
 [
     AC_ARG_ENABLE([background],
-	AS_HELP_STRING([--enable-background], [Support for background file operations [[yes]]]),
-	[
-	    if test "x$enableval" = xno; then
-		enable_background=no
-	    else
-		enable_background=yes
-	    fi
-	],
-	[enable_background=yes])
+    AS_HELP_STRING([--enable-background], [Support for background file operations [yes]]),
+    [
+        if test "x$enableval" = xno; then
+        enable_background=no
+        else
+        enable_background=yes
+        fi
+    ],
+    [enable_background=yes])
 
     if test "x$enable_background" = xyes; then
         AC_DEFINE(ENABLE_BACKGROUND, 1, [Define to enable background file operations])