configure: Support empty target list (--target-list=)

Specifying an empty target list with --target-list= is shorter
than specifying --disable-user --disable-system.

Both variants should give the same result: no targets at all.

This modification implements that feature.

It uses a trick which works with POSIX compliant shells to test whether
target_list is undefined (=> default targets) or empty (=> no targets).
Signed-off-by: NStefan Weil <sw@weilnetz.de>
Signed-off-by: NStefan Hajnoczi <stefanha@gmail.com>

configure

@@ -125,7 +125,8 @@ cc_i386=i386-pc-linux-gnu-gcc
 libs_qga=""
 debug_info="yes"
 
-target_list=""
+# Don't accept a target_list environment variable.
+unset target_list
 
 # Default value for a variable defining feature "foo".
 #  * foo="no"  feature will only be used if --enable-foo arg is given
@@ -1288,7 +1289,7 @@ if ! "$python" -c 'import sys; sys.exit(sys.version_info < (2,4) or sys.version_
   exit 1
 fi
 
-if test -z "$target_list" ; then
+if test -z "${target_list+xxx}" ; then
     target_list="$default_target_list"
 else
     target_list=`echo "$target_list" | sed -e 's/,/ /g'`