configure: fall back to using full path if src is a directory

Or when building in-tree.
Also don't try to remove src on distclean in these cases.
Signed-off-by: Andreas Cadhalpun <Andreas.Cadhalpun@googlemail.com>

Makefile

@@ -181,7 +181,10 @@ clean::
 
 distclean::
 	$(RM) $(DISTCLEANSUFFIXES)
-	$(RM) config.* .config libavutil/avconfig.h .version avversion.h version.h libavutil/ffversion.h libavcodec/codec_names.h src
+	$(RM) config.* .config libavutil/avconfig.h .version avversion.h version.h libavutil/ffversion.h libavcodec/codec_names.h
+ifeq ($(SRC_LINK),src)
+	$(RM) src
+endif
 	$(RM) -rf doc/examples/pc-uninstalled
 
 config:

configure

@@ -4710,7 +4710,7 @@ link_name=$(mktemp -u $TMPDIR/name_XXXXXXXX)
 mkdir "$link_dest"
 $ln_s "$link_dest" "$link_name"
 touch "$link_dest/test_file"
-if [ -e "$link_name/test_file" ]; then
+if [ "$source_path" != "." ] && ([ ! -d src ] || [ -L src ]) && [ -e "$link_name/test_file" ]; then
     # create link to source path
     [ -e src ] && rm src
     $ln_s "$source_path" src