build: don't let a large sparse temporary file cause "make dist" to fail

* bootstrap.conf (bootstrap_epilogue): Replace the offending grep
command from po/Makefile.in.in's $(DOMAIN).pot-update rule.

bootstrap.conf

@@ -337,4 +337,15 @@ bootstrap_epilogue()
   m=gnulib-tests/gnulib.mk
   sed 's,\.\./\.\.,..,g' $m > $m-t
   mv -f $m-t $m
+
+  # Since this is a "GNU" package, replace this line
+  #   if LC_ALL=C grep 'GNU @PACKAGE@' $(top_srcdir)/* 2>/dev/null \
+  #      | grep -v 'libtool:' >/dev/null; then
+  # with this:
+  #   if true; then
+  # Why?  That pipeline searches all files in $(top_srcdir), and if you
+  # happen to have large files (or apparently large sparse files), the
+  # first grep may well run out of memory.
+  perl -pi -e 's/if LC_ALL=C grep .GNU .PACKAGE.*; then/if true; then/' \
+    po/Makefile.in.in
 }