improve configure auto-magic

- Makefile and prepare-source.mak should not keep a configure.sh output from a failed
  autoconf run
- the configure wrapper tool should always call prepare-source to rebuild configure.sh
  when required
- Makefile and prepare-source.mak should rebuild aclocal.m4 when configure.ac changes

Makefile.in

@@ -143,10 +143,13 @@ gensend: gen
 conf:
 	cd $(srcdir) && $(MAKE) -f prepare-source.mak conf
 
+aclocal.m4: configure.ac
+	aclocal -I m4
+
 configure.sh config.h.in: configure.ac aclocal.m4
 	@if test -f configure.sh; then cp -p configure.sh configure.sh.old; else touch configure.sh.old; fi
 	@if test -f config.h.in; then cp -p config.h.in config.h.in.old; else touch config.h.in.old; fi
-	autoconf -o configure.sh
+	autoconf -o configure.sh || ( rm configure.sh ; false )
 	autoheader && touch config.h.in
 	@if diff configure.sh configure.sh.old >/dev/null 2>&1; then \
 	    echo "configure.sh is unchanged."; \

configure

@@ -15,13 +15,13 @@ if test ! -f "$realconfigure"; then
     else
 	actions='build'
     fi
-    if "$dir/prepare-source" $actions; then
-	:
-    else
-	echo 'Failed to build configure.sh and/or config.h.in -- giving up.' >&2
-	rm -f "$realconfigure"
-	exit 1
-    fi
+fi
+if "$dir/prepare-source" $actions; then
+    :
+else
+    echo 'Failed to build configure.sh and/or config.h.in -- giving up.' >&2
+    rm -f "$realconfigure"
+    exit 1
 fi
 
 exec "$realconfigure" "${@}"

prepare-source.mak

 conf: configure.sh config.h.in
 
+aclocal.m4: configure.ac
+	aclocal -I m4
+
 configure.sh: configure.ac aclocal.m4
-	autoconf -o configure.sh
+	autoconf -o configure.sh || ( rm configure.sh ; false )
 
 config.h.in: configure.ac aclocal.m4
 	autoheader && touch config.h.in