O_CLOEXEC needs _GNU_SOURCE defined

O_CLOEXEC needs _GNU_SOURCE defined

From man open(2):

    The O_CLOEXEC, O_DIRECTORY, and O_NOFOLLOW flags are not specified
    in POSIX.1-2001, but are specified in POSIX.1-2008.  Since glibc
    2.12, one can obtain their definitions by defining either
    _POSIX_C_SOURCE with a value greater than or equal to 200809L or
    _XOPEN_SOURCE with a value greater than or equal to 700.  In glibc
    2.11 and earlier, one obtains the definitions by defining
    _GNU_SOURCE.

And indeed, with the uClibc C library, O_CLOEXEC is not exposed if
_GNU_SOURCE is not defined. Therefore, this commit fixes the build of
gstreamer-vaapi with the uClibc C library.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
https://bugzilla.gnome.org/show_bug.cgi?id=779953