Fix ranlib check when cross-compiling

Without this patch /usr/bin/ranlib is tested for "-D", not ranlib from the
cross-compiler suite being in use.

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>

diff -uNr ffmpeg-2.6.2.org/configure ffmpeg-2.6.2/configure
--- ffmpeg-2.6.2.org/configure	2015-04-12 00:47:55.000000000 +0200
+++ ffmpeg-2.6.2/configure	2015-04-16 21:53:24.313202517 +0200
@@ -2719,11 +2719,6 @@
 nm_default="nm -g"
 objformat="elf"
 pkg_config_default=pkg-config
-if ranlib 2>&1 | grep -q "\-D "; then
-    ranlib_default="ranlib -D"
-else
-    ranlib_default="ranlib"
-fi
 strip_default="strip"
 yasmexe_default="yasm"
 windres_default="windres"
@@ -3001,6 +2996,12 @@
         die "Must specify target arch and OS when cross-compiling"
 fi
 
+if "${cross_prefix}ranlib" 2>&1 | grep -q "\-D "; then
+    ranlib_default="ranlib -D"
+else
+    ranlib_default="ranlib"
+fi
+
 test -n "$valgrind" && toolchain="valgrind-memcheck"
 
 case "$toolchain" in