build: Allow autogen.sh to be run from another directory
build: Allow autogen.sh to be run from another directory

This allows the build pattern:
   git clone …
   mkdir /tmp/build
   cd /tmp/build
   /path/to/checkout/autogen.sh
   make
which is useful for builddir ≠ srcdir setups, such as buildbots.

lathiat/avahi,lathiat/avahi,heftig/avahi,lathiat/avahi,heftig/avahi,heftig/avahi,heftig/avahi,lathiat/avahi,heftig/avahi,lathiat/avahi,lathiat/avahi,heftig/avahi