libwebsockets: fix build with nothread toolchains

libwebsockets: fix build with nothread toolchains

>From upstream changelog:
> If LWS_MAX_SMP=1, then there is no code related to pthreads compiled in the
  library. If more than 1, a small amount of pthread mutex code is built into
  the library.

Set LWS_MAX_SMP=1 for toolchains without thread support, otherwise leave it
unset so upstreams CMakeLists.txt will take care of it.

Fixes:
http://autobuild.buildroot.net/results/dbf/dbf45a52b9aef602fd62ad1ca10957bafe656923/
http://autobuild.buildroot.net/results/26c/26c2ac28f44f8f1b227caa715af60bcd92a5f740/
http://autobuild.buildroot.net/results/d65/d65dcbfb96b249e01b034c01a52884b522c778b3/
http://autobuild.buildroot.net/results/e55/e550ebfdd1e8f459ca7b12f2708a704eaec52474/
.. and more.

Signed-off-by: Jörg Krause <joerg.krause@embedded.rocks>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>