Don't set POSIX_SOURCE on FreeBSD.
/wrkdirs/usr/ports/deskutils/kdepim-runtime/work/kdepim-runtime-17.07.80/resources/imap/saslplugin/xoauth2plugin.c:178:5:
error: implicitly declaring library function 'snprintf' with type 'int (char *, unsigned long, const char *, ...)' [-Werror,-Wimplicit-function-declaration] snprintf(context->out_buf, context->out_buf_len, "user=%s\1auth=Bearer %s\1\1", authid, token); ^
/wrkdirs/usr/ports/deskutils/kdepim-runtime/work/kdepim-runtime-17.07.80/resources/imap/saslplugin/xoauth2plugin.c:178:5:
note: include the header <stdio.h> or explicitly provide a declaration for 'snprintf'
1 error generated.
Differential Revision: https://phabricator.kde.org/D6926