Fix the dynamic loading issue on NetBSD

On NetBSD, it is not supported to load a library linked against
libpthread into a program that wasn't (since the C library provides
stubs for some pthread-functions, which might have already been called
before libpthread is loaded, leading to problems).

Patch from Matthias Drochner

m4/ax_pthread.m4

@@ -162,6 +162,11 @@ case ${host_os} in
         darwin*)
         ax_pthread_flags="-pthread $ax_pthread_flags"
         ;;
+	netbsd*)
+	# use libc stubs, don't link against libpthread, to allow
+	# dynamic loading
+	ax_pthread_flags=""
+	;;
 esac
 
 # Clang doesn't consider unrecognized options an error unless we specify