build: fix 'make distcheck' with pdwtags installed

I am getting this failure with 'make distcheck':

  GEN    ../../src/remote_protocol-structs
/bin/sh: ../../src/remote_protocol-structs-t: Permission denied
make[4]: *** [../../src/remote_protocol-structs] Error 1

since it attempts a sub-run of a VPATH 'make check' where $(srcdir)
is intentionally read-only.  I'm not sure which commit introduced
the problem, although I suspect it was around 62dee6f when I
refactored protocol struct checking to be more powerful.

$(@F) is required by POSIX, and although it is not yet portable
to all make implementations, we already require GNU make.

* src/Makefile.am (PDWTAGS): Generate temp file into current
directory, since $(srcdir) is read-only during distcheck.

src/Makefile.am

@@ -255,9 +255,9 @@ PDWTAGS = \
 		-e '    exit 8;'					\
 		-e '  }'						\
 		-e '}'							\
-		> $@-t;							\
+		> $(@F)-t;						\
 	  case $$? in 8) exit 0;; 0) ;; *) exit 1;; esac;		\
-	  diff -u $@-t $@; st=$$?; rm -f $@-t; exit $$st;	\
+	  diff -u $(@F)-t $@; st=$$?; rm -f $(@F)-t; exit $$st;		\
 	else								\
 	  echo 'WARNING: you lack pdwtags; skipping the $@ test' >&2;	\
 	  echo 'WARNING: install the dwarves package to get pdwtags' >&2; \