configure: do not allow nvenc to be build on cygwin to avoid _WIN32 definition hack

Suggested-by: James Almer <jamrial@gmail.com>
Reviewed-by: James Almer <jamrial@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

configure

@@ -4934,7 +4934,8 @@ enabled libxavs           && require libxavs xavs.h xavs_encoder_encode -lxavs
 enabled libxvid           && require libxvid xvid.h xvid_global -lxvidcore
 enabled libzmq            && require_pkg_config libzmq zmq.h zmq_ctx_new
 enabled libzvbi           && require libzvbi libzvbi.h vbi_decoder_new -lzvbi
-enabled nvenc             && { check_header nvEncodeAPI.h || die "ERROR: nvEncodeAPI.h not found."; }
+enabled nvenc             && { check_header nvEncodeAPI.h || die "ERROR: nvEncodeAPI.h not found."; } &&
+                             { [ $target_os != cygwin ] || die "ERROR: NVENC is not supported on Cygwin currently."; }
 enabled openal            && { { for al_libs in "${OPENAL_LIBS}" "-lopenal" "-lOpenAL32"; do
                                check_lib 'AL/al.h' alGetError "${al_libs}" && break; done } ||
                                die "ERROR: openal not found"; } &&