bug#26570: GC_is_heap_ptr() dep for 2.2.1

[Zefram]

Compilation of 2.2.1 fails for me, producing a lot of warnings about
implicit declaration of GC_is_heap_ptr(), and ultimately

  CCLD     guile
./.libs/libguile-2.2.so: undefined reference to `GC_is_heap_ptr'
collect2: error: ld returned 1 exit status
Makefile:2439: recipe for target 'guile' failed
make[3]: *** [guile] Error 1

At a guess, maybe this is supposed to be supplied by libgc.  But I have
the version of libgc that README says is required (7.2), and configure
was happy with it.  Maybe a higher version is now required, and README
and configure need updating?

-zefram