getresuid.2, mremap.2, poll.2, remap_file_pages.2, setresuid.2, splice.2, sync_file_r...

getresuid.2, mremap.2, poll.2, remap_file_pages.2, setresuid.2, splice.2, sync_file_range.2, syscall.2, tee.2, vmsplice.2, INFINITY.3, aio_init.3, asprintf.3, assert_perror.3, basename.3, bsd_signal.3, canonicalize_file_name.3, clog10.3, crypt.3, dl_iterate_phdr.3, dlopen.3, encrypt.3, exp10.3, fcloseall.3, fenv.3, fopencookie.3, ftw.3, getaddrinfo_a.3, getloadavg.3, getutent.3, grantpt.3, hsearch.3, lseek64.3, memmem.3, mempcpy.3, pow10.3, program_invocation_name.3, ptsname.3, putgrent.3, sched_getcpu.3, sincos.3, strchr.3, strfry.3, strnlen.3, strptime.3, strstr.3, strverscmp.3, swab.3, sysv_signal.3, tsearch.3, unlockpt.3, wcwidth.3: Add reference to feature_test_macros(7)

Some pages simply list feature test macro requirements in
the form:

    #define #GNU_SOURCE
    #include <someheader.h>

For these pages, add a "See feature_test_macros(7)" comment
on the "#define" line.

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>