xfce-git.bbclass initial add

common preparations for xfce-git-repos

* implement autogen.sh call in do_configure_prepend
* AUTOTOOLS_COPYACLOCAL = "1": In oe-core commit
  79ea036de331bde65a88fb777647dc099ef05acf copying aclocals was moved to
  do_configure[prefuncs]. These are called before do_configure_prepend which
  causes configure.ac not yet being created and autotools_copy_aclocals
  returning undone.

By using this class tons of errors like

| aclocal: error: couldn't open directory '/home/a.mueller/tmp/oe-core-eglibc/work/armv7at2hf-vfp-neon-angstrom-linux-gnueabi/xfce4-settings/4.11.0+gitAUTOINC+b7a0e1fd77-r0/git/aclocal-copy/': No such file or directory
| autoreconf: aclocal failed with exit status: 1

are fixed.

Signed-off-by: Andreas Müller <schnitzeltony@googlemail.com>