bug#30714: Racket doesn't build any more, xform error

[Christopher Lemmer Webber]

Related to the attempt to upgrade Racket to 6.12:
  https://debbugs.gnu.org/cgi/bugreport.cgi?bug=30603

Apparently the patch I proposed may break GC, so another option should
be found.  But more interestingly is that my thought that things broke
because of a change from Racket 6.11 to 6.12 appears to be wrong... I am
also getting a similar error in trying to build Racket 6.11.  I assume
this must be a dependency change but I don't yet know which:

  env XFORM_USE_PRECOMP=xsrc/precomp.h ../racketcgc  -cqu ./xform.rkt --setup . 
--depends --cpp "gcc -E -I./.. -I./../include -I./../../rktio -I../../rktio -g 
-O2  -pthread   -DUSE_SENORA_GC   "  --keep-lines -o xsrc/number.c 
./../src/number.c
  Error [GCING] 1780 in ./../src/number.c: Function minus_zero_p declared 
__xform_nongcing__, but includes a function call at __signbitf128.
  Error [GCING] 1958 in ./../src/number.c: Function double_is_integer declared 
__xform_nongcing__, but includes a function call at __isinff128.
  Error [GCING] 4050 in ./../src/number.c: Function double_fits_fixnum declared 
__xform_nongcing__, but includes a function call at __isinff128.
  xform: Errors converting
    context...:
     
/tmp/guix-build-racket-6.11.drv-0/racket-6.11/src/racket/gc2/xform-collects/compiler/private/xfor...:8:2:
 xform
     
/tmp/guix-build-racket-6.11.drv-0/racket-6.11/src/racket/gc2/xform-collects/xform/xform-mod.rkt:
 [running body]
     /tmp/guix-build-racket-6.11.drv-0/racket-6.11/src/racket/gc2/xform.rkt: 
[running body]
  make[4]: *** [Makefile:268: xsrc/number.c] Error 1

It seems that Racket last built successfully in late December:

  https://hydra.gnu.org/job/gnu/master/racket-6.11.x86_64-linux