py-google-cloud-sdk: Choose a safe fallback for Python interpreter

The logic to find a Python executable can be summarized as:
 - CLOUDSDK_PYTHON environment variable defined:
    + If it matches `*python2*', use it
 - Otherwise:
    + Try `python2'
    + Fallback to `python'

Substitute the latter fallback to ${PYTHONBIN} in order to have a
safe choice.

Should fix PR pkg/52122 reported by <jklos>.
This patch is based on the one proposed by <maya> (but restricted
to CLOUDSDK_PYTHON lines and the fallback `python').

Bump PKGREVISION.

Index: Makefile
===================================================================
RCS file: /cvsroot/pkgsrc/net/py-google-cloud-sdk/Makefile,v
retrieving revision 1.3
diff -u -p -r1.3 Makefile
--- Makefile	2 Apr 2016 08:26:49 -0000	1.3
+++ Makefile	7 Jun 2018 07:22:33 -0000
@@ -3,6 +3,7 @@
 GCLOUDVER=	96.0.0
 DISTNAME=	google-cloud-sdk-${GCLOUDVER}-linux-x86
 PKGNAME=	${PYPKGPREFIX}-google-cloud-sdk-${GCLOUDVER}
+PKGREVISION=	1
 CATEGORIES=	net
 MASTER_SITES=	https://dl.google.com/dl/cloudsdk/channels/rapid/downloads/
 
@@ -23,6 +24,13 @@ REPLACE_PYTHON+=	bin/bootstrapping/*
 PLIST_SUBST+=		PYPKGPREFIX=${PYPKGPREFIX}
 MESSAGE_SUBST+=		PYPKGPREFIX=${PYPKGPREFIX}
 
+SUBST_CLASSES+=		python
+SUBST_STAGE.python=	pre-configure
+SUBST_MESSAGE.python=	Replacing hard coded python executable name
+SUBST_FILES.python=	lib/googlecloudsdk/core/*
+SUBST_FILES.python+=	bin/*
+SUBST_SED.python=	-e "/CLOUDSDK_PYTHON=/ s,python$$,${PYTHONBIN},"
+
 INSTALLATION_DIRS=	${PKGBASE}
 
 do-install: