How to change python version in makefile - compiling Multicorn (PostgreSQL FDW extension)

I'm trying to install PostgreSQL extension Multicorn on CentOS 6.5. The problem that I have though is that default version of python on Centos is 2.6 and Multicorn requires 2.7 or 3.3. I'm trying to compile Multicorn using this tutorial, but it's a little dated and the step where python version is changed doesn't work anymore:

sed -i 's/^PYEXEC = python$/PYEXEC = python2.7/' Makefile

Can someone help me to make the above command work again, or show me how to edit the makefile to change the version of python? I can call python version 2.7 in the command line with python2.7. The version 2.6 is called with just python - apparently I can't change that without breaking CentOS.

This is the makefile:

MODULE_big   = multicorn
OBJS         =  src/errors.o src/python.o src/query.o src/multicorn.o


DATA         = $(filter-out $(wildcard sql/*--*.sql),$(wildcard sql/*.sql))

DOCS         = $(wildcard doc/*.md)

EXTENSION    = multicorn
EXTVERSION   = $(shell grep default_version $(EXTENSION).control | sed -e "s/default_version[[:space:]]*=[[:space:]]*'\([^']*\)'/\1/")

all: preflight-check sql/$(EXTENSION)--$(EXTVERSION).sql

install: python_code 

sql/$(EXTENSION)--$(EXTVERSION).sql: sql/$(EXTENSION).sql
    cp $< $@

preflight-check:
    ./preflight-check.sh


python_code: setup.py
    cp ./setup.py ./setup--$(EXTVERSION).py
    sed -i -e "s/__VERSION__/$(EXTVERSION)-dev/g" ./setup--$(EXTVERSION).py
    $(PYTHON) ./setup--$(EXTVERSION).py install
    rm ./setup--$(EXTVERSION).py

release-zip: all
    git archive --format zip --prefix=multicorn-$(EXTVERSION)/ --output ./multicorn-$(EXTVERSION).zip HEAD
    unzip ./multicorn-$(EXTVERSION).zip
    rm ./multicorn-$(EXTVERSION).zip
    sed -i -e "s/__VERSION__/$(EXTVERSION)/g"  ./multicorn-$(EXTVERSION)/META.json  ./multicorn-$(EXTVERSION)/setup.py  ./multicorn-$(EXTVERSION)/python/multicorn/__init__.py
    zip -r ./multicorn-$(EXTVERSION).zip ./multicorn-$(EXTVERSION)/
    rm ./multicorn-$(EXTVERSION) -rf

coverage:
    lcov -d . -c -o lcov.info
    genhtml --show-details --legend --output-directory=coverage --title=PostgreSQL --num-spaces=4 --prefix=./src/ `find . -name lcov.info -print`

DATA = sql/$(EXTENSION)--$(EXTVERSION).sql
EXTRA_CLEAN = sql/$(EXTENSION)--$(EXTVERSION).sql ./multicorn-$(EXTVERSION).zip
PG_CONFIG ?= pg_config
PGXS := $(shell $(PG_CONFIG) --pgxs)
REGRESS      = virtual_tests

include $(PGXS)

with_python_no_override = no

ifeq ($(with_python),yes)
    with_python_no_override = yes
endif

ifdef PYTHON_OVERRIDE
    with_python_no_override = no
endif


ifeq ($(with_python_no_override),yes)
    SHLIB_LINK = $(python_libspec) $(python_additional_libs) $(filter -lintl,$(LIBS))
    override CPPFLAGS := -I. -I$(srcdir) $(python_includespec) $(CPPFLAGS)
    override PYTHON = python${python_version}
else
    ifdef PYTHON_OVERRIDE
        override PYTHON = ${PYTHON_OVERRIDE}
    endif

    ifeq (${PYTHON}, )
        override PYTHON = python
    endif


    python_version = $(shell ${PYTHON} --version 2>&1 | cut -d ' ' -f 2 | cut -d '.' -f 1-2)
    PYTHON_CONFIG ?= python${python_version}-config

    PY_LIBSPEC = $(shell ${PYTHON_CONFIG} --libs)
    PY_INCLUDESPEC = $(shell ${PYTHON_CONFIG} --includes)
    PY_CFLAGS = $(shell ${PYTHON_CONFIG} --cflags)
    PY_LDFLAGS = $(shell ${PYTHON_CONFIG} --ldflags)
    SHLIB_LINK = $(PY_LIBSPEC) $(PY_LDFLAGS) $(PY_ADDITIONAL_LIBS) $(filter -lintl,$(LIBS))
    override PG_CPPFLAGS  := $(PY_INCLUDESPEC) $(PG_CPPFLAGS)
    override CPPFLAGS := $(PG_CPPFLAGS) $(CPPFLAGS)
endif


PYTHON_TEST_VERSION ?= $(python_version)
PG_TEST_VERSION ?= $(MAJORVERSION)
SUPPORTS_WRITE=$(shell expr ${PG_TEST_VERSION} \>= 9.3)

TESTS        = $(wildcard test-$(PYTHON_TEST_VERSION)/sql/multicorn*.sql)
ifeq (${SUPPORTS_WRITE}, 1)
  TESTS += $(wildcard test-$(PYTHON_TEST_VERSION)/sql/write*.sql)
endif
REGRESS      = $(patsubst test-$(PYTHON_TEST_VERSION)/sql/%.sql,%,$(TESTS))
REGRESS_OPTS = --inputdir=test-$(PYTHON_TEST_VERSION) --load-language=plpgsql

$(info Python version is $(python_version))

Solution

The best practice is to run make as

PYTHON=python2.7 make

If you take a look at line 26 of your Makefile, you'll see that compilation is handled by setup.py script that is invoked by executable specified in $(PYTHON) variable which you can override by setting it from environment. Another way to do this (e.g. if you want to do multiple builds) is this one:

export PYTHON=python2.7
make

Changing script behavior by enviroment variables or command line arguments is more reasonable and often more simple than patching script source itself.

More about Makefile variables: http://ftp.gnu.org/old-gnu/Manuals/make-3.79.1/html_chapter/make_6.html#SEC68