sqlite: fix build without threads

sqlite: fix build without threads

If threads are not available, set SQLITE_THREADSAFE to 0.
Indeed, since version 3.25.0, the following line:
THREADSAFE_FLAGS=-DSQLITE_THREADSAFE=0
has been removed from configure.ac
As a result, SQLITE_THREADSAFE will be set to a default value of 1 even
if --disable-threadsafe is set

Fixes:
 - http://autobuild.buildroot.org/results/70b1582edcb787746e4483e80b27f86ac781f0fa

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>