configure: Avoid compiling system tools on user build by default

User-mode does not need the system tools. Do not build them by
default if the user specifies --disable-system.

This disables building the following binaries on a user-only build:

- elf2dmp
- qemu-edid
- qemu-ga
- qemu-img
- qemu-io
- qemu-nbd
- ivshmem-client
- ivshmem-server

The qemu-user binaries are not affected by this change.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20200217133327.25144-1-f4bug@amsat.org>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>

configure

@@ -455,7 +455,7 @@ guest_agent_ntddscsi="no"
 guest_agent_msi=""
 vss_win32_sdk=""
 win_sdk="no"
-want_tools="yes"
+want_tools=""
 libiscsi=""
 libnfs=""
 coroutine=""
@@ -2213,6 +2213,16 @@ else
     echo big/little test failed
 fi
 
+##########################################
+# system tools
+if test -z "$want_tools"; then
+    if test "$softmmu" = "no"; then
+        want_tools=no
+    else
+        want_tools=yes
+    fi
+fi
+
 ##########################################
 # cocoa implies not SDL or GTK
 # (the cocoa UI code currently assumes it is always the active UI