This adds a new switch to autoconf, --enable-mapreduce, to connect
the GPMapreduce extension to the build. If set to on the mapreduce
extension in gpAux/extensions will be built and installed.  As a
side effect this will properly clean and distclean for gpfdist in
gpAux/extensions as well. Autoconf checks for libyaml since it's
a depedency for GPMapreduce.

This is the first stab at gpmapreduce, once in we can start moving
gpmapreduce back into src/bin and move the mapreduce test suite in
under gpAux/extensions.