Having 'CFLAGS="-pipe -Werror -Wall -Wextra -O3 -fno-fat-lto-objects -flto" CXXFLAGS="-pipe -Wall -Wextra -O3 -fno-fat-lto-objects -flto" LDFLAGS="-Wl,-O1,-s -flto=12" ' in config.site and running "./configure" with gcc8 prints:

checking for fchdir... no
checking for fsync... no
checking for fdatasync... no

and config.log has:

configure:11440: checking for fchdir
configure:11453: gcc -c -pipe -Werror -Wall -Wextra -O3 -fno-fat-lto-objects -flto  conftest.c >&5
conftest.c: In function 'main':
conftest.c:250:7: error: unused variable 'x' [-Werror=unused-variable]
 void *x=fchdir
       ^
cc1: all warnings being treated as errors
configure:11453: $? = 1
configure: failed program was:
| /* confdefs.h */
...
| #define HAVE_DIRFD 1
| /* end confdefs.h.  */
| #include <unistd.h>
| int
| main ()
| {
| void *x=fchdir
|   ;
|   return 0;
| }
configure:11460: result: no