Update Copyright notices for 2014
Update configure.ng for autoconf 2.67

We already require GNU autoconf 2.67 for generating our build system (at
least this is what the INSTALL document states), so update the build system
accordingly and implement all changes that autoupdate(1) suggests:

 - Update AC_PREREQ and AC_INIT
 - Use AC_LINK_IFELSE, AC_RUN_IFELSE, and AC_COMPILE_IFELSE
 - Remove AC_TYPE_SIGNAL (we don't use RETSIGTYPE)