Only add -Wno-stringop-truncation on supported compilers

Only add -Wno-stringop-truncation on supported compilers

The -Wno-stringop-truncation flag is only supported by some GCC versions
and not by Clang (macOS, FreeBSD) at all.

Move the includes to the top the file to have them available when running
the check_c_compiler_flag.

Change-Id: I452bc4ee935d13f8e9095d0a31805a3bbaff0cec
Signed-off-by: Arne Schwabe <arne@rfc2549.org>
Acked-by: Frank Lichtenheld <frank@lichtenheld.com>
Message-Id: <20230629215611.3292788-3-arne@rfc2549.org>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg26791.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>