Using #if instead of #ifdef was causing some issues when cross-compiling, like:
../ext/curl/gstcurlsmtpsink.c:54:5: error: "HAVE_SYS_SOCKET_H" is not
defined, evaluates to 0 [-Werror=undef]
54 | #if HAVE_SYS_SOCKET_H
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/8589>