pgxs and bison, flex

There are some extensions that build with pgxs that use bison and flex.
Their makefiles are set up to use the variables BISON and FLEX that pgxs
provides.  Except that that depends on how PostgreSQL was built.  A
binary package that was built in a clean chroot would probably not have
those variables set, because the programs were not present in the build
process.  There have been a number of bugs related to those extensions
because of that.

I propose that we apply the attached patch to make sure those variables
are set to a usable default value in any case.