configure.ac: Move AS_LIBTOOL() and SCXI_SO_VERSION
Move the call to AS_LIBTOOL() to avoid warnings from automake 1.12.6.
Move SCXI_SO_VERSION as it is nearby!
On Gentoo Linux:
$ WANT_AUTOMAKE=1.12 ./autogen.sh
configure.ac:101: warning: LT_INIT was called before AM_PROG_AR
/usr/share/aclocal-1.12/ar-lib.m4:13: AM_PROG_AR is expanded from...
configure.ac:101: the top level
configure.ac:101: warning: AC_PROG_LIBTOOL was called before AM_PROG_AR
/usr/share/aclocal-1.12/ar-lib.m4:13: AM_PROG_AR is expanded from...
configure.ac:101: the top level
configure.ac:101: warning: LT_INIT was called before AM_PROG_AR
aclocal.m4:8669: AM_PROG_AR is expanded from...
configure.ac:101: the top level
configure.ac:101: warning: AC_PROG_LIBTOOL was called before AM_PROG_AR
aclocal.m4:8669: AM_PROG_AR is expanded from...
configure.ac:101: the top level