apidoc: Limit INPUT to src subdirectory and README.md

While 0909bf6c explicitly includes the whole source tree (to cover README.md),
this has the unpleasant side effect of covering a workspace under "testing"
with all its sources, or any other potential subdirectory that exists.

Makefile.am

@@ -34,7 +34,7 @@ Doxyfile :	Doxyfile.in
 		sed \
 		-e "s:\@PACKAGE_VERSION\@:$(PACKAGE_VERSION):" \
 		-e "s:\@PACKAGE_NAME\@:$(PACKAGE_NAME):" \
-		-e "s:\@SRC_DIR\@:$(srcdir):" \
+		-e "s:\@SRC_DIR\@:$(srcdir):g" \
 		$(srcdir)/$@.in > $@
 
 apidoc :	Doxyfile