lang/python/tool.mk: allow setting the desired executable name

[nia <nia%NetBSD.org@localhost>]

Trying to make tool.mk more useful for resolving problems such as this:

http://us-east.manta.joyent.com/pkgsrc/public/reports/Darwin/trunk/x86_64/20191112.2308/MesaLib-19.2.3/configure.log

ok?

Index: tool.mk
===================================================================
RCS file: /cvsroot/pkgsrc/lang/python/tool.mk,v
retrieving revision 1.1
diff -u -r1.1 tool.mk
--- tool.mk	14 Oct 2011 08:55:54 -0000	1.1
+++ tool.mk	15 Nov 2019 15:00:28 -0000
@@ -3,14 +3,24 @@
 # Create `python' interpreter wrapper for applicable Python bin.
 #
 # This mk fragment can be included in all packages that uses `python'
-# as a tool without version suffix.
+# as a tool without a pkgsrc-style version suffix.
 #
 # Keywords: python
 #
+# === Package-settable variables ===
+#
+# PYTHON_TOOL
+#	The name of the Python executable the package expects to need.
+#
+#	Possible values: python3, python2
+#	Default: python
+#
 
 .if !defined(PYTHON_TOOL_MK)
 PYTHON_TOOL_MK=	defined
 
+PYTHON_TOOL?=	python
+
 .if !defined(PYTHON_PYVERSION_MK)
 .include "../../lang/python/pyversion.mk"
 .endif
@@ -21,7 +31,7 @@
 buildlink-bin-python:
 	${RUN} \
 	f="${PYTHONBIN}"; \
-	t="${BUILDLINK_DIR}/bin/python"; \
+	t="${BUILDLINK_DIR}/bin/${PYTHON_TOOL}"; \
 	if ${TEST} -f $$f -a ! -f $$t ; then \
 		${LN} -sf $$f $$t; \
 	fi