Rename core directory because of autoconf name clash

While doing the checks configure might generate "core" files and will
then try to remove them. Having a "core" directory generates an error
while runing the configure script.

There's no workaround but to rename the core directory.
Implemented autotools based build scripts

supported options:

--with-dependency-search=DIR    will tell configure to look for various
dependencies in DIR/include and DIR/lib

Alternatively you can also specify libsodium header and libs location
with --with-libsodium-headers and --with-libsodium-libs if it is
installed elsewhere.

Ncurses and libconfig is handled via the default pkg-config way, see
./configure --help=short for detailed information.

The cmake build still works.