PATCH pass PGOPTIONS to pg_regress

Hi,

attached patch passes PGOPTIONS env variable to pg_regress in pgxs.mk

This is especially useful when developing extensions for different postgres versions
where some session_variables might or might not exists.

Consider something like this in an extensions makefile:

ifeq ($(shell test $(VERSION_NUM) -ge 90600; echo $$?),0)
PGOPTIONS+= "--max_parallel_workers_per_gather=0"
endif

But also when there are many testfiles it might be convenient to align some session parameter globally
In the Makefile.

Thoughts?

Cheers

Manuel Kniep