packaging: be explicit about Python version in rpm spec

Fedora 31 has Python3 at /usr/bin/python ... but expect everybody to not just
find python in $PATH but be explicit about whether they want python2 or
python3. mercurial.spec just used 'python' and would fail when it unknowingly
used Python 3 and ended up with Mercurial setup.py reporting "Python 3.7
detected." and talking about the HGPYTHON3 environment variable.

For now, just be explicit about using system python2 as python executable when
building rpms.

contrib/packaging/mercurial.spec

@@ -1,164 +1,167 b''
 %global emacs_lispdir %{_datadir}/emacs/site-lisp
 
 %define withpython %{nil}
 
 %if "%{?withpython}"
 
 %global pythonver %{withpython}
 %global pythonname Python-%{withpython}
 %global docutilsname docutils-0.14
 %global docutilsmd5 c53768d63db3873b7d452833553469de
 %global pythonhg python-hg
 %global hgpyprefix /opt/%{pythonhg}
 # byte compilation will fail on some some Python /test/ files
 %global _python_bytecompile_errors_terminate_build 0
 
 %else
 
-%global pythonver %(python -c 'import sys;print(".".join(map(str, sys.version_info[:2])))')
+%global pythonexe python2
+%global pythonver %(%{pythonexe} -c 'import sys;print(".".join(map(str, sys.version_info[:2])))')
 
 %endif
 
 Summary: A fast, lightweight Source Control Management system
 Name: mercurial
 Version: snapshot
 Release: 0
 License: GPLv2+
 Group: Development/Tools
 URL: https://mercurial-scm.org/
 Source0: %{name}-%{version}-%{release}.tar.gz
 %if "%{?withpython}"
 Source1: %{pythonname}.tgz
 Source2: %{docutilsname}.tar.gz
 %endif
 BuildRoot: %{_tmppath}/%{name}-%{version}-%{release}-root
 
 BuildRequires: make, gcc, gettext
 %if "%{?withpython}"
 BuildRequires: readline-devel, openssl-devel, ncurses-devel, zlib-devel, bzip2-devel
 %else
 BuildRequires: python >= 2.7, python-devel, python-docutils >= 0.5
 Requires: python >= 2.7
 %endif
 # The hgk extension uses the wish tcl interpreter, but we don't enforce it
 #Requires: tk
 
 %description
 Mercurial is a fast, lightweight source control management system designed
 for efficient handling of very large distributed projects.
 
 %prep
 
 %if "%{?withpython}"
 %setup -q -n mercurial-%{version}-%{release} -a1 -a2
 # despite the comments in cgi.py, we do this to prevent rpmdeps from picking /usr/local/bin/python up
-sed -i '1c#! /usr/bin/env python' %{pythonname}/Lib/cgi.py
+sed -i '1c#! /usr/bin/env %{pythonexe}' %{pythonname}/Lib/cgi.py
 %else
 %setup -q -n mercurial-%{version}-%{release}
 %endif
 
 %build
 
 %if "%{?withpython}"
 
 PYPATH=$PWD/%{pythonname}
 cd $PYPATH
 ./configure --prefix=%{hgpyprefix}
 make all %{?_smp_mflags}
 cd -
 
 cd %{docutilsname}
 LD_LIBRARY_PATH=$PYPATH $PYPATH/python setup.py build
 cd -
 
 # verify Python environment
 LD_LIBRARY_PATH=$PYPATH PYTHONPATH=$PWD/%{docutilsname} $PYPATH/python -c 'import sys, zlib, bz2, ssl, curses, readline'
 
 # set environment for make
 export PATH=$PYPATH:$PATH
 export LD_LIBRARY_PATH=$PYPATH
 export CFLAGS="-L $PYPATH"
 export PYTHONPATH=$PWD/%{docutilsname}
 
 %endif
 
-make all
+make all PYTHON=%{pythonexe}
 make -C contrib/chg
 
+sed -i -e '1s|#!/usr/bin/env python$|#!/usr/bin/env %{pythonexe}|' contrib/hg-ssh
+
 %install
 rm -rf $RPM_BUILD_ROOT
 
 %if "%{?withpython}"
 
 PYPATH=$PWD/%{pythonname}
 cd $PYPATH
 make install DESTDIR=$RPM_BUILD_ROOT
 # these .a are not necessary and they are readonly and strip fails - kill them!
 rm -f %{buildroot}%{hgpyprefix}/lib/{,python2.*/config}/libpython2.*.a
 cd -
 
 cd %{docutilsname}
 LD_LIBRARY_PATH=$PYPATH $PYPATH/python setup.py install --root="$RPM_BUILD_ROOT"
 cd -
 
-PATH=$PYPATH:$PATH LD_LIBRARY_PATH=$PYPATH make install DESTDIR=$RPM_BUILD_ROOT PREFIX=%{hgpyprefix} MANDIR=%{_mandir}
+PATH=$PYPATH:$PATH LD_LIBRARY_PATH=$PYPATH make install PYTHON=%{pythonexe} DESTDIR=$RPM_BUILD_ROOT PREFIX=%{hgpyprefix} MANDIR=%{_mandir}
 mkdir -p $RPM_BUILD_ROOT%{_bindir}
 ( cd $RPM_BUILD_ROOT%{_bindir}/ && ln -s ../..%{hgpyprefix}/bin/hg . )
 ( cd $RPM_BUILD_ROOT%{_bindir}/ && ln -s ../..%{hgpyprefix}/bin/python2.? %{pythonhg} )
 
 %else
 
-make install DESTDIR=$RPM_BUILD_ROOT PREFIX=%{_prefix} MANDIR=%{_mandir}
+make install PYTHON=%{pythonexe} DESTDIR=$RPM_BUILD_ROOT PREFIX=%{_prefix} MANDIR=%{_mandir}
 
 %endif
 
 install -m 755 contrib/chg/chg $RPM_BUILD_ROOT%{_bindir}/
 install -m 755 contrib/hgk $RPM_BUILD_ROOT%{_bindir}/
 install -m 755 contrib/hg-ssh $RPM_BUILD_ROOT%{_bindir}/
 
 bash_completion_dir=$RPM_BUILD_ROOT%{_sysconfdir}/bash_completion.d
 mkdir -p $bash_completion_dir
 install -m 644 contrib/bash_completion $bash_completion_dir/mercurial.sh
 
 zsh_completion_dir=$RPM_BUILD_ROOT%{_datadir}/zsh/site-functions
 mkdir -p $zsh_completion_dir
 install -m 644 contrib/zsh_completion $zsh_completion_dir/_mercurial
 
 mkdir -p $RPM_BUILD_ROOT%{emacs_lispdir}
 install -m 644 contrib/mercurial.el $RPM_BUILD_ROOT%{emacs_lispdir}/
 install -m 644 contrib/mq.el $RPM_BUILD_ROOT%{emacs_lispdir}/
 
 mkdir -p $RPM_BUILD_ROOT/%{_sysconfdir}/mercurial/hgrc.d
 
 %clean
 rm -rf $RPM_BUILD_ROOT
 
 %files
 %defattr(-,root,root,-)
 %doc CONTRIBUTORS COPYING doc/README doc/hg*.txt doc/hg*.html *.cgi contrib/*.fcgi
 %doc %attr(644,root,root) %{_mandir}/man?/hg*
 %doc %attr(644,root,root) contrib/*.svg
 %dir %{_datadir}/zsh/
 %dir %{_datadir}/zsh/site-functions/
 %{_datadir}/zsh/site-functions/_mercurial
 %dir %{_datadir}/emacs/site-lisp/
 %{_datadir}/emacs/site-lisp/mercurial.el
 %{_datadir}/emacs/site-lisp/mq.el
 %{_bindir}/hg
 %{_bindir}/chg
 %{_bindir}/hgk
 %{_bindir}/hg-ssh
 %dir %{_sysconfdir}/bash_completion.d/
 %config(noreplace) %{_sysconfdir}/bash_completion.d/mercurial.sh
 %dir %{_sysconfdir}/mercurial
 %dir %{_sysconfdir}/mercurial/hgrc.d
 %if "%{?withpython}"
 %{_bindir}/%{pythonhg}
 %{hgpyprefix}
 %else
 %{_libdir}/python%{pythonver}/site-packages/%{name}-*-py%{pythonver}.egg-info
 %{_libdir}/python%{pythonver}/site-packages/%{name}
 %{_libdir}/python%{pythonver}/site-packages/hgext
 %{_libdir}/python%{pythonver}/site-packages/hgext3rd
 %{_libdir}/python%{pythonver}/site-packages/hgdemandimport
 %endif