berkeleydb: accommodate threads presence/absence

In BerkeleyDB the most convenient implementation of mutexes on Linux is
via POSIX mutex, and that requires pthread library.

Still it is possible to build (and hopefully use) BerkeleyDB without
mutexes. For this we pass "--disable-mutexsupport" during configuration
of the package.

Fixes:
http://autobuild.buildroot.net/results/717f3b37600a56262badc6f7cb64d7949fdacb67/
http://autobuild.buildroot.net/results/80ebf0382992b277fd94743815bbf0c7426a3654/



Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>