configure: require --arch and --target-os when cross-compiling

Originally committed as revision 21766 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -1427,12 +1427,12 @@ yasmexe="yasm"
 nm_opts='-g'
 
 # machine
-arch=$(uname -m)
+arch_default=$(uname -m)
 cpu="generic"
 
 # OS
-target_os=$(tolower $(uname -s))
-host_os=$target_os
+target_os_default=$(tolower $(uname -s))
+host_os=$target_os_default
 
 # configurable options
 enable avcodec
@@ -1606,6 +1606,13 @@ set >> $logfile
 
 test -n "$cross_prefix" && enable cross_compile
 
+if enabled cross_compile; then
+    test -n "$arch" && test -n "$target_os" ||
+        die "Must specify target arch and OS when cross-compiling"
+fi
+
+set_default arch target_os
+
 ar="${cross_prefix}${ar}"
 cc_default="${cross_prefix}${cc_default}"
 nm_default="${cross_prefix}${nm_default}"