configure: linux-user doesn't need neither fdt nor slirp

if softmmu is not enabled, we disable by default fdt and
slirp as they are only used by -softmmu targets.

A side effect is the git submodules are not cloned
if they are not needed.

Clone and build can be forced with --enable-fdt and
--enable-slirp.

Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20190621130544.18860-1-lvivier@redhat.com>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>

configure

@@ -4135,6 +4135,11 @@ elif test "$fdt" != "yes" ; then
   fdt=no
 fi
 
+# fdt is only required when building softmmu targets
+if test -z "$fdt" -a "$softmmu" != "yes" ; then
+    fdt="no"
+fi
+
 if test "$fdt" != "no" ; then
   fdt_libs="-lfdt"
   # explicitly check for libfdt_env.h as it is missing in some stable installs
@@ -6078,6 +6083,11 @@ fi
 ##########################################
 # check for slirp
 
+# slirp is only required when building softmmu targets
+if test -z "$slirp" -a "$softmmu" != "yes" ; then
+    slirp="no"
+fi
+
 case "$slirp" in
   "" | yes)
     if $pkg_config slirp; then