Add ability to disable build of all targets

Allow passing of '--target-list=' to configure to request that
all targets are to be disabled. This allows for doing a very
fast tools-only build of things like qemu-img, qemu-io, qemu-nbd.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

configure

@@ -127,7 +127,7 @@ cc_i386=i386-pc-linux-gnu-gcc
 libs_qga=""
 debug_info="yes"
 
-target_list=""
+target_list="DEFAULT"
 
 # Default value for a variable defining feature "foo".
 #  * foo="no"  feature will only be used if --enable-foo arg is given
@@ -1317,15 +1317,10 @@ if ! "$python" -c 'import sys; sys.exit(sys.version_info < (2,4) or sys.version_
   exit 1
 fi
 
-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
+if test "$target_list" = "DEFAULT" ; then
+    target_list=`echo "$default_target_list" | sed -e 's/,/ /g'`
 fi
+
 # see if system emulation was really requested
 case " $target_list " in
   *"-softmmu "*) softmmu=yes