Allow building resiprocate from an external directory

...ource top dir.   This patch allows building from an external directory so that object files and other files generated by automake and autoconf do not pollute the source tree.

Example:
# git clone https://github.com/resiprocate/resiprocate
# cd resiprocate
# autoreconf -if
# cd ..
# mkdir BUILD
# cd BUILD
# ../resiprocate/configure --prefix=`pwd`/build
# make && make install