Comment 1 for bug 1595562

Update to C++11

As per https://bugs.launchpad.net/ecap/+bug/1595488 it seems that
moving to requiring C++11 in line with squid is necessary.

- Enforce C++11 requirement using autoconf-archive macros.
- Remove trace of std::tr1.
- Replace deprecated auto_ptr with unique_ptr.
- As you already depend on pkg-config to find libecap, do
  the same for libclamav. (If you prefer, I could go the other
  way and replace pkg-config with real autoconf detection for
  libecap.)
- add ax_pthread.m4 to fix missing macros.