provider/postgres: Allow brew installed postgresql

This commit modifies the GNUmakefile used for starting PostgreSQL for
testing purposes to prefer the version installed via Homebrew, followed
by the version installed via MacPorts. POSTGRES, PSQL and INITDB may be
overriden as Make variables if neither of these package management
systems is in use.

builtin/providers/postgresql/GNUmakefile

@@ -5,14 +5,15 @@
 # may change in the future and is likely not required when doing
 # non-delete related operations. But for now it is.
 
-POSTGRES?=/opt/local/lib/postgresql96/bin/postgres
-PSQL?=/opt/local/lib/postgresql96/bin/psql
+POSTGRES?=$(wildcard /usr/local/bin/postgres /opt/local/lib/postgresql96/bin/postgres)
+PSQL?=$(wildcard /usr/local/bin/psql /opt/local/lib/postgresql96/bin/psql)
+INITDB?=$(wildcard /usr/local/bin/initdb /opt/local/lib/postgresql96/bin/initdb)
 
 PGDATA?=$(GOPATH)/src/github.com/hashicorp/terraform/builtin/providers/postgresql/data
 
 initdb::
 	echo "" > pwfile
-	/opt/local/lib/postgresql96/bin/initdb --no-locale -U postgres -A md5 --pwfile=pwfile -D $(PGDATA)
+	$(INITDB) --no-locale -U postgres -A md5 --pwfile=pwfile -D $(PGDATA)
 
 startdb::
 	2>&1 \