* .gitlab-ci.yml: Add Libmicrohttpd package for CI testing
  using Debian/gcc, Fedora/clang and MinGW64
* .travis.sh: Add Libmicrohttpd package for OSX/clang
* .travis_setup.sh: Add Libmicrohttpd package for Debian/gcc
  and clang
* README.md: Add info which mentioning Libmicrohttpd as
  optional requirement
* configure.ac: Check for Libmicrohttpd availability