PY and PELICAN env variable overrides for Make

This allows the user to set PY and/or PELICAN environment variable
overrides, which will be respected by the Makefile and devserver.sh.
If, for example, the default Python on your system were Python 3 and
you wanted to run Make with Python 2, using bash you could run
`PY=python2 make`. Refs #915.

docs/getting_started.rst

@@ -271,6 +271,12 @@ use rsync over ssh::
 
 That's it! Your site should now be live.
 
+(The default ``Makefile`` and ``devserver.sh`` scripts use the ``python`` and
+``pelican`` executables to complete its tasks. If you want to use different
+executables, such as ``python3``, you can set the ``PY`` and ``PELICAN``
+environment variables, respectively, to override the default executable names.)
+
+
 Writing content using Pelican
 =============================
 

pelican/tools/pelican_quickstart.py

@@ -274,9 +274,9 @@ needed by Pelican.
         try:
             with codecs.open(os.path.join(CONF['basedir'], 'Makefile'), 'w', 'utf-8') as fd:
                 mkfile_template_name = 'Makefile'
-                py_v = 'PY=python'
+                py_v = 'PY?=python'
                 if six.PY3:
-                    py_v = 'PY=python3'
+                    py_v = 'PY?=python3'
                 template = string.Template(py_v)
                 fd.write(template.safe_substitute(CONF))
                 fd.write('\n')
@@ -296,9 +296,9 @@ needed by Pelican.
         try:
             with codecs.open(os.path.join(CONF['basedir'], 'develop_server.sh'), 'w', 'utf-8') as fd:
                 lines = list(get_template('develop_server.sh'))
-                py_v = 'PY=python\n'
+                py_v = 'PY=${PY:-python}\n'
                 if six.PY3:
-                    py_v = 'PY=python3\n'
+                    py_v = 'PY=${PY:-python3}\n'
                 lines = lines[:4] + [py_v] + lines[4:]
                 for line in lines:
                     template = string.Template(line)

pelican/tools/templates/Makefile.in

@@ -1,4 +1,4 @@
-PELICAN=$pelican
+PELICAN?=$pelican
 PELICANOPTS=$pelicanopts
 
 BASEDIR=$$(CURDIR)

pelican/tools/templates/develop_server.sh.in

@@ -2,7 +2,7 @@
 ##
 # This section should match your Makefile
 ##
-PELICAN=$pelican
+PELICAN=$${PELICAN:-$pelican}
 PELICANOPTS=$pelicanopts
 
 BASEDIR=$$(pwd)