Enforce dependency on libpq version >= 9.1

PGRES_COPY_BOTH was introduced in 9.1: we can ifdef the hell out of
pgpath, but we may as well bury the dead horses instead of beating
them.

They smell funny, too.

doc/src/install.rst

@@ -19,7 +19,8 @@ The current `!psycopg2` implementation supports:
 
 - Python 2 versions from 2.6 to 2.7
 - Python 3 versions from 3.1 to 3.5
-- PostgreSQL versions from 7.4 to 9.4
+- PostgreSQL server versions from 7.4 to 9.5
+- PostgreSQL client library version from 9.1
 
 .. _PostgreSQL: http://www.postgresql.org/
 .. _Python: http://www.python.org/

psycopg/pqpath.c

@@ -1913,7 +1913,7 @@ pq_fetch(cursorObject *curs, int no_result)
         break;
 
     default:
-        /* PGRES_COPY_BOTH, PGRES_SINGLE_TUPLE, future statuses */
+        /* PGRES_SINGLE_TUPLE, future statuses */
         Dprintf("pq_fetch: got unsupported result: status = %d pgconn = %p",
             pgstatus, curs->conn);
         PyErr_Format(NotSupportedError,

psycopg/psycopg.h

@@ -26,6 +26,10 @@
 #ifndef PSYCOPG_H
 #define PSYCOPG_H 1
 
+#if PG_VERSION_NUM < 90100
+#error "Psycopg requires PostgreSQL client library (libpq) >= 9.1"
+#endif
+
 #define PY_SSIZE_T_CLEAN
 #include <Python.h>
 #include <libpq-fe.h>