Hello,

Running configure with  --enable-checking=stringoverrun has no effect in that GC_CHECK_STRING_OVERRUN is not defined in the resulting config.h file.

I believe I know why this is happening.  On line 278, ac_gc_check_string_overrun is set when the stringoverrun option is specified.  Then on line 297 is the following test.
  if test x$ac_gc_check_stringoverrun != x ; then
(Note that there is no _ between string and overrun).

I believe that all that is necessary to fix this is to change line 297 to the following, but I cannot test it because I do not have the correct version of autoconf installed.
  if test x$ac_gc_check_string_overrun != x ; then

Please let me know if I should go ahead and make this change.