sqlite: do not assume thread support is available

The --enable-threadsafe option was unconditionally passed, without
taking into account whether the underlying toolchain had thread
support or not. Now, we properly pass --enable-threadsafe or
--disable-threadsafe depending on the availability of thread support.

Fixes
http://autobuild.buildroot.net/results/defdc098cff293baabf2f89587476eba71f1c0d0/build-end.log

.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>