Move AC_LANG usage below AC_INIT

The previous positioning resulted in a failure with autoconf 2.63:

configure.ac:17: error: m4_defn: undefined macro: _m4_divert_diversion
configure.ac:17: the top level
autom4te: /usr/bin/m4 failed with exit status: 1
Merge pull request #1 from schuay/master

Move AC_LANG usage below AC_INIT