* getrandom was added in glibc 2.25
https://sourceware.org/bugzilla/show_bug.cgi?id=17252

* copy_file_range was added in glibc 2.27
https://sourceware.org/git/?p=glibc.git;a=commit;h=bad7a0c81f501fbbcc79af9eaa4b8254441c4a1f

* threads.h should not exist for glibc < 2.28

* single_threaded.h should not exist for glibc < 2.35

* Apply suggestions from code review

Co-authored-by: Alex Rønne Petersen <alex@alexrp.com>

* 2.35 instead of 35

* before 2.35 instead of 2.34 and before

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Co-authored-by: Alex Rønne Petersen <alex@alexrp.com>