Add meson build system
some simlifications have been done:
- use .pc files for libgcrypt and libgpg-error
- _WIN32_WINNT is not set anymore: it's a user option, not a build one
- remove useless header checks and inclusion
- remove HAVE_CONFIG_H as config.h is always built*
- remove HAVE_LIBGPG_ERROR as libgpg-error is mandatory
- remove HAVE_PTHREAD_H as pthread is mandatory on non Windows OS