configure: Move up the avbuild directory creation

The early check for inconsistent in-source vs out-of-source build
cannot generate a config.log otherwise.

(Cherry-picked from libav commit 0ee78020)
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>
Signed-off-by: James Almer <jamrial@gmail.com>

configure

@@ -3350,6 +3350,8 @@ target_path='$(CURDIR)'
 DEPCMD='$(DEP$(1)) $(DEP$(1)FLAGS) $($(1)DEP_FLAGS) $< 2>/dev/null | sed -e "/^\#.*/d" -e "s,^[[:space:]]*$(@F),$(@D)/$(@F)," > $(@:.o=.d)'
 DEPFLAGS='-MM'
 
+mkdir -p ffbuild
+
 # find source path
 if test -f configure; then
     source_path=.
@@ -3595,7 +3597,6 @@ disable_components(){
 
 map 'disable_components $v' $LIBRARY_LIST
 
-mkdir -p ffbuild
 echo "# $0 $FFMPEG_CONFIGURATION" > $logfile
 set >> $logfile