man/man7/feature_test_macros.7: Drop _ISOC9X_SOURCE

man/man7/feature_test_macros.7: Drop _ISOC9X_SOURCE

_ISOC9X_SOURCE was a temporary macro for use before the final name
of the standard was known.  It became obsolete when _ISOC99_SOURCE
was added in glibc 2.1.3 (1999), and has not been recognized since
_ISOC11_SOURCE was added in glibc 2.16 (2012).[1]

[1] <https://sourceware.org/git/?p=glibc.git;a=commitdiff;h=d78099052b6b>

Signed-off-by: Mark Harris <mark.hsj@gmail.com>
Message-ID: <bf785830f727b9fd7a40001b861b743f88fe2a7a.1769581651.git.mark.hsj@gmail.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>