libcurl: fix build without threads

libcurl: fix build without threads

When c-ares is not enabled libcurl enables the threaded DNS resolver by
default. Make sure the threaded resolvers is disabled when the toolchain
does not support threads.

Add upstream patch that fixes the configure option for disabling the
threaded resolver.

Fixes:
http://autobuild.buildroot.net/results/39f/39fa63fb2ecb75e4b2521d1ee3dfa357c4e5c594/
http://autobuild.buildroot.net/results/dfd/dfd296086d0d6bed73b92fe2fa4ba5434dddf796/

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>