Revert "Add ability to disable build of all targets"

This reverts commit 66d5499b3754b83c09487259c08fe2ce73188a59.

This commit broke --target-list="x86_64-softmmu" and the fix isn't immediatley
obvious.

Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

configure

@@ -126,7 +126,7 @@ cc_i386=i386-pc-linux-gnu-gcc
 libs_qga=""
 debug_info="yes"
 
-target_list="DEFAULT"
+target_list=""
 
 # Default value for a variable defining feature "foo".
 #  * foo="no"  feature will only be used if --enable-foo arg is given
@@ -1319,10 +1319,15 @@ if ! "$python" -c 'import sys; sys.exit(sys.version_info < (2,4) or sys.version_
   exit 1
 fi
 
-if test "$target_list" = "DEFAULT" ; then
-    target_list=`echo "$default_target_list" | sed -e 's/,/ /g'`
+if test -z "$target_list" ; then
+    target_list="$default_target_list"
+else
+    target_list=`echo "$target_list" | sed -e 's/,/ /g'`
+fi
+if test -z "$target_list" ; then
+    echo "No targets enabled"
+    exit 1
 fi
-
 # see if system emulation was really requested
 case " $target_list " in
   *"-softmmu "*) softmmu=yes