Make trickle multithread version signal safe

Because, select(),poll(), read(), write() and many other overloaded functions are
normally reentrant from signal handlers, we need to block all signals while we hold
mutex or else we can deadlock.

Modified Makefile.am because while make did not build libtrickle.so,
'make install' was trying to install it. I'm really shooting in the dark for
autoconf problem so maybe it is not the best way to resolve the problem.