Remove deprecated autoconf macros

GNOME_COMMON_INIT is deprecated since 2010 and definetely obsolete in
eog since commit 08aca771ca.
AC_ISC_POSIX is not needed anymore on modern systems.
Modernize configure.ac

- Require libtool 2.2
  Nobody seems to use libtool 1.x anymore, allows dropping more deprecated
  libtool macros
- AC_HELP_STRING -> AS_HELP_STRING
- AC_TRY_CPP/LINK -> AC_PREPROC/LINK
- Try making the indentation more consistent