first of all we need to create directory for config files outside $PGDATA:
in my case this is /u01/postgres/9.5/main/conf/

next we need to modify service and add option with path of postgresql.conf

vim /usr/lib/systemd/system/postgresql-9.5.service

[Unit]
Description=PostgreSQL 9.5 database server
After=syslog.target
After=network.target

[Service]
Type=forking

User=postgres
Group=postgres

# Note: avoid inserting whitespace in these Environment= lines, or you may
# break postgresql-setup.

# Location of database directory
Environment=PGDATA=/u01/postgres/9.5/main/pgdata

# Where to send early-startup messages from the server (before the logging
# options of postgresql.conf take effect)
# This is normally controlled by the global default set by systemd
# StandardOutput=syslog

# Disable OOM kill on the postmaster
OOMScoreAdjust=-1000

ExecStartPre=/usr/pgsql-9.5/bin/postgresql95-check-db-dir ${PGDATA}
ExecStart=/usr/pgsql-9.5/bin/pg_ctl start -D ${PGDATA} -s -w -t 300 -o "-c config_file=/u01/postgres/9.5/main/conf/postgresql.conf"
ExecStop=/usr/pgsql-9.5/bin/pg_ctl stop -D ${PGDATA} -s -m fast
ExecReload=/usr/pgsql-9.5/bin/pg_ctl reload -D ${PGDATA} -s

# Give a reasonable amount of time for the server to start up/shut down
TimeoutSec=300

[Install]
WantedBy=multi-user.target

in config file we modify location of other config files:

data_directory='/u01/postgres/9.5/main/pgdata'
hba_file='/u01/postgres/9.5/main/conf/pg_hba.conf'
ident_file='/u01/postgres/9.5/main/conf/pg_ident.conf'