have_func: checking for onig_region_memsize() in ruby.h... -------------------- yes

"gcc -o conftest -I../../.ext/include/x86_64-linux -I../.././include -I../.././ext/strscan -I../../.    -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/strscan -I../../.    -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"
conftest.c: In function ‘t’:
conftest.c:16:57: error: ‘onig_region_memsize’ undeclared (first use in this function)
 int t(void) { void ((*volatile p)()); p = (void ((*)()))onig_region_memsize; return !p; }
                                                         ^~~~~~~~~~~~~~~~~~~
conftest.c:16:57: note: each undeclared identifier is reported only once for each function it appears in
checked program was:
/* begin */
 1: #include "ruby.h"
 2: 
 3: #include <ruby.h>
 4: 
 5: /*top*/
 6: extern int t(void);
 7: int main(int argc, char **argv)
 8: {
 9:   if (argc > 1000000) {
10:     int (* volatile tp)(void)=(int (*)(void))&t;
11:     printf("%d", (*tp)());
12:   }
13: 
14:   return !!argv[argc];
15: }
16: int t(void) { void ((*volatile p)()); p = (void ((*)()))onig_region_memsize; return !p; }
/* end */

"gcc -o conftest -I../../.ext/include/x86_64-linux -I../.././include -I../.././ext/strscan -I../../.    -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: #include <ruby.h>
 4: 
 5: /*top*/
 6: extern int t(void);
 7: int main(int argc, char **argv)
 8: {
 9:   if (argc > 1000000) {
10:     int (* volatile tp)(void)=(int (*)(void))&t;
11:     printf("%d", (*tp)());
12:   }
13: 
14:   return !!argv[argc];
15: }
16: extern void onig_region_memsize();
17: int t(void) { onig_region_memsize(); return 0; }
/* end */

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

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