autogen.sh: explicitly build its containing directory

No changes in currently-accepted usages (no-argument), but allows
  /src/path/autogen.sh && /src/path/configure
for simpler out-of-tree builds

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #13316

autogen.sh

@@ -1,4 +1,2 @@
 #!/bin/sh
-
-autoreconf -fiv || exit 1
-rm -Rf autom4te.cache
+autoreconf -fiv "$(dirname "$0")" && rm -rf "$(dirname "$0")"/autom4te.cache