src/configure has mangled libffcall test

As of current master (9405c097), it looks like something's gone wrong while generating src/configure - around line 59800, there's the following:

# Check whether --with-libffcall-prefix was given.
if test ${with_libffcall_prefix+y}
then :
  withval=$with_libffcall_prefix; libcallback in DIR/include and DIR/lib
  --without-libffcall-prefix     don't search for libavcall
else case e in #(
  e) libcallback in includedir and libdir ;;
esac

(i.e. it starts out with the code to test for the configure arg, then cuts into the middle of a help message, which produces lots of complaints when running the script if you don't have libffcall installed). Regenerating configure myself with autoconf 2.72 and m4 1.4.19 produces the same result.

It looks like something's maybe gone wrong with the expansion of AC_LIB_LINKFLAGS_BODY, causing it to repeat a chunk of text?