cmake: always enable GNU and recent-POSIX extensions, like autoconf does

cmake: always enable GNU and recent-POSIX extensions, like autoconf does

If the C library is (e)glibc, this allows use of POSIX, BSD, SVID, GNU,
etc., extensions to ISO C, regardless of using -ansi or not.

Not doing this broke the cmake build on Linux since commit 18b08180,
which added AC_USE_SYSTEM_EXTENSIONS to configure.ac (and removed
_GNU_SOURCE from files that use it) without also updating the cmake
build system. SO_PEERCRED is defined unconditionally, but struct ucred
is considered to be a GNU extension, so can't be used under _GNU_SOURCE.

Bug: https://bugs.freedesktop.org/show_bug.cgi?id=29228
Reviewed-by: Will Thompson <will.thompson@collabora.co.uk>