configure: do not set bsd_user/linux_user early

Similar to other optional features, leave the variables empty and compute
the actual value later.  Use the existence of include or source directories
to detect whether an OS or CPU supports respectively bsd-user and linux-user.

For now, BSD user-mode emulation is buildable even on TCI-only
architectures.  This probably will change once safe signals are
brought over from linux-user.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

configure

@@ -320,8 +320,8 @@ linux="no"
 solaris="no"
 profiler="no"
 softmmu="yes"
-linux_user="no"
-bsd_user="no"
+linux_user=""
+bsd_user=""
 pkgversion=""
 pie=""
 qom_cast_debug="yes"
@@ -538,7 +538,6 @@ gnu/kfreebsd)
 ;;
 freebsd)
   bsd="yes"
-  bsd_user="yes"
   make="${MAKE-gmake}"
   # needed for kinfo_getvmmap(3) in libutil.h
 ;;
@@ -583,7 +582,6 @@ haiku)
 ;;
 linux)
   linux="yes"
-  linux_user="yes"
   vhost_user=${default_feature:-yes}
 ;;
 esac
@@ -1259,18 +1257,26 @@ if eval test -z "\${cross_cc_$cpu}"; then
     cross_cc_vars="$cross_cc_vars cross_cc_${cpu}"
 fi
 
-# For user-mode emulation the host arch has to be one we explicitly
-# support, even if we're using TCI.
-if [ "$ARCH" = "unknown" ]; then
-  bsd_user="no"
-  linux_user="no"
-fi
-
 default_target_list=""
 deprecated_targets_list=ppc64abi32-linux-user
 deprecated_features=""
 mak_wilds=""
 
+if [ "$linux_user" != no ]; then
+    if [ "$targetos" = linux ] && [ -d $source_path/linux-user/include/host/$cpu ]; then
+        linux_user=yes
+    elif [ "$linux_user" = yes ]; then
+        error_exit "linux-user not supported on this architecture"
+    fi
+fi
+if [ "$bsd_user" != no ]; then
+    if [ "$bsd_user" = "" ]; then
+        test $targetos = freebsd && bsd_user=yes
+    fi
+    if [ "$bsd_user" = yes ] && ! [ -d $source_path/bsd-user/$targetos ]; then
+        error_exit "bsd-user not supported on this host OS"
+    fi
+fi
 if [ "$softmmu" = "yes" ]; then
     mak_wilds="${mak_wilds} $source_path/configs/targets/*-softmmu.mak"
 fi