Try to install Python scripts with versioned Python shebangs

Allow overriding the Python version.

The Python setuptools based build system already modifies shebangs.

The Python scripts have unversioned Python shebangs so they
will fail if an unversioned Python is not provided by the OS.

The Debian bullseye release will not have an unversioned Python by default.

Avoid make pattern rules and shell functions since they are not portable.

Makefile

@@ -3,7 +3,28 @@ prefix := $(HOME)
 bindir := $(prefix)/bin
 mandir := $(prefix)/share/man/man1
 
-all: doc
+all: build doc
+
+build:
+	if [ -n "$$PYTHON" ] && "$$PYTHON" -c 'import mercurial' 2> /dev/null ; then \
+		: Use chosen Python version ; \
+	elif python3 -c 'import mercurial' 2> /dev/null ; then \
+		PYTHON=python3 ; \
+	elif python2 -c 'import mercurial' 2> /dev/null ; then \
+		PYTHON=python2 ; \
+	elif python -c 'import mercurial' 2> /dev/null ; then \
+		PYTHON=python ; \
+	fi ; \
+	if [ -n "$$PYTHON" ] ; then \
+		PYTHON=python ; \
+	fi ; \
+	mkdir -p bin ; \
+	for s in git-remote-hg git-hg-helper ; do \
+		printf "%s\n" "#!/usr/bin/env $$PYTHON" > "bin/$$s" ; \
+		tail -n +2 "./$$s" >> "bin/$$s" ; \
+		chmod 755 "bin/$$s" ; \
+		touch -r "./$$s" "bin/$$s" ; \
+	done
 
 doc: doc/git-remote-hg.1
 
@@ -15,13 +36,14 @@ doc/git-remote-hg.1: doc/git-remote-hg.txt
 
 clean:
 	$(RM) doc/git-remote-hg.1
+	$(RM) -r bin/
 
 D = $(DESTDIR)
 
-install:
+install: build
 	install -d -m 755 $(D)$(bindir)/
-	install -m 755 git-remote-hg $(D)$(bindir)/git-remote-hg
-	install -m 755 git-hg-helper $(D)$(bindir)/git-hg-helper
+	install -m 755 bin/git-remote-hg $(D)$(bindir)/git-remote-hg
+	install -m 755 bin/git-hg-helper $(D)$(bindir)/git-hg-helper
 
 install-doc: doc
 	install -d -m 755 $(D)$(mandir)/
@@ -39,4 +61,4 @@ pypi-upload:
 pypi-test:
 	twine upload --repository-url https://test.pypi.org/legacy/ dist/*
 
-.PHONY: all test install install-doc clean pypy pypy-upload
+.PHONY: all build test install install-doc clean pypy pypy-upload