have_func: checking for rb_ary_subseq()... -------------------- yes

"gcc -o conftest -I../../../.ext/include/x86_64-linux -I../../.././include -I../../.././ext/racc/cparse    -g -O2 -fPIC conftest.c  -L. -L../../.. -L. -fstack-protector-strong -rdynamic -Wl,-export-dynamic     -Wl,-rpath,/usr/local/rvm/rubies/ruby-2.7.4/lib -L/usr/local/rvm/rubies/ruby-2.7.4/lib -lruby-static -lz -lpthread -lrt -lrt -ldl -lcrypt -lm   -lm   -lc"
checked program was:
/* begin */
1: #include "ruby.h"
2: 
3: int main(int argc, char **argv)
4: {
5:   return !!argv[argc];
6: }
/* end */

"gcc -o conftest -I../../../.ext/include/x86_64-linux -I../../.././include -I../../.././ext/racc/cparse    -g -O2 -fPIC conftest.c  -L. -L../../.. -L. -fstack-protector-strong -rdynamic -Wl,-export-dynamic     -Wl,-rpath,/usr/local/rvm/rubies/ruby-2.7.4/lib -L/usr/local/rvm/rubies/ruby-2.7.4/lib -lruby-static -lz -lpthread -lrt -lrt -ldl -lcrypt -lm   -lm   -lc"
checked program was:
/* begin */
 1: #include "ruby.h"
 2: 
 3: /*top*/
 4: extern int t(void);
 5: int main(int argc, char **argv)
 6: {
 7:   if (argc > 1000000) {
 8:     int (* volatile tp)(void)=(int (*)(void))&t;
 9:     printf("%d", (*tp)());
10:   }
11: 
12:   return !!argv[argc];
13: }
14: int t(void) { void ((*volatile p)()); p = (void ((*)()))rb_ary_subseq; return !p; }
/* end */

--------------------

extconf.h is:
/* begin */
1: #ifndef EXTCONF_H
2: #define EXTCONF_H
3: #define HAVE_RB_ARY_SUBSEQ 1
4: #endif
/* end */