Added autotools and libtool support

These changes add autotools and libtool support to erfa. Among the benefits are:

  * A familiar way to build the library: ./configure && make && make install
  * Shared and static libraries in all the systems supported by libtool
  * Automatic targets to run the tests (make check), to create the tarball (make dist)
    and to install (make install), among others

Upstream has to decide a versioning scheme for the releases of erfa (numeric, by date, etc) and include it in the macro AC_INIT in configure.ac. The tarball will be named according to that scheme