scripts/sphinx-pre-install: add minimum support for RHEL

RHEL 7.x and clone distros are shipped with Sphinx 1.1.x,
with is incompatible with Kernel ReST markups.

So, on those systems, the only alternative is to install
it via a Python virtual environment.

While seeking for "pip" on CentOS 7.3, I noticed that it
is not really needed, as python-virtualenv has its version
packaged there already. So, remove this from the list of
requirements for all distributions.

With regards to PDF, we need at least texlive-tabulary
extension, but that is not shipped there (at least on
CentOS). So, disable PDF packages as a whole.

Please notice, however, that texlive + amsmath is needed for
ReST to properly handle ReST ".. math::" tags. Yet, Sphinx
fall back to display the LaTeX math expressions as-is, if
such extension is not available.

So, let's just disable all texlive packages as a whole.

Signed-off-by: Mauro Carvalho Chehab <[e-mail]>
Signed-off-by: Jonathan Corbet <[e-mail]>

scripts/sphinx-pre-install

···
 		$prog = findprog("virtualenv-3.5") if (!$prog);
 
 		check_program("virtualenv", 0) if (!$prog);
-		check_program("pip", 0) if (!findprog("pip3"));
 		$need_sphinx = 1;
 	} else {
 		add_package("python-sphinx", 0);
···
 		"python-sphinx"		=> "python3-sphinx",
 		"sphinx_rtd_theme"	=> "python3-sphinx-rtd-theme",
 		"virtualenv"		=> "virtualenv",
-		"pip"			=> "python3-pip",
 		"dot"			=> "graphviz",
 		"convert"		=> "imagemagick",
 		"Pod::Usage"		=> "perl-modules",
···
 		"python-sphinx"		=> "python3-sphinx",
 		"sphinx_rtd_theme"	=> "python3-sphinx_rtd_theme",
 		"virtualenv"		=> "python3-virtualenv",
-		"pip"			=> "python3-pip",
 		"dot"			=> "graphviz",
 		"convert"		=> "ImageMagick",
 		"Pod::Usage"		=> "perl-Pod-Usage",
···
 		"dejavu-sans-mono-fonts",
 	);
 
+	#
+	# Checks valid for RHEL/CentOS version 7.x.
+	#
+	if (! $system_release =~ /Fedora/) {
+		$map{"virtualenv"} = "python-virtualenv";
+	}
+
 	my $release;
 
 	$release = $1 if ($system_release =~ /Fedora\s+release\s+(\d+)/);
···
 	check_missing(\%map);
 
 	return if (!$need && !$optional);
-	printf("You should run:\n\n\tsudo dnf install -y $install\n");
+
+	if ($release >= 18) {
+		# dnf, for Fedora 18+
+		printf("You should run:\n\n\tsudo dnf install -y $install\n");
+	} else {
+		# yum, for RHEL (and clones) or Fedora version < 18
+		printf("You should run:\n\n\tsudo yum install -y $install\n");
+	}
 }
 
 sub give_opensuse_hints()
···
 		"python-sphinx"		=> "python3-sphinx",
 		"sphinx_rtd_theme"	=> "python3-sphinx_rtd_theme",
 		"virtualenv"		=> "python3-virtualenv",
-		"pip"			=> "python3-pip",
 		"dot"			=> "graphviz",
 		"convert"		=> "ImageMagick",
 		"Pod::Usage"		=> "perl-Pod-Usage",
···
 		"python-sphinx"		=> "python3-sphinx",
 		"sphinx_rtd_theme"	=> "python3-sphinx_rtd_theme",
 		"virtualenv"		=> "python3-virtualenv",
-		"pip"			=> "python3-pip",
 		"dot"			=> "graphviz",
 		"convert"		=> "ImageMagick",
 		"Pod::Usage"		=> "perl-Pod-Usage",
···
 	my @tex_pkgs = (
 		"texlive-fontsextra",
 	);
-
-	my $release;
 
 	check_rpm_missing(\@tex_pkgs, 1) if ($pdf);
 	check_missing(\%map);
···
 	my %map = (
 		"sphinx_rtd_theme"	=> "python-sphinx_rtd_theme",
 		"virtualenv"		=> "python-virtualenv",
-		"pip"			=> "python-pip",
 		"dot"			=> "graphviz",
 		"convert"		=> "imagemagick",
 		"xelatex"		=> "texlive-bin",
···
 	my %map = (
 		"sphinx_rtd_theme"	=> "dev-python/sphinx_rtd_theme",
 		"virtualenv"		=> "dev-python/virtualenv",
-		"pip"			=> "dev-python/pip",
 		"dot"			=> "media-gfx/graphviz",
 		"convert"		=> "media-gfx/imagemagick",
 		"xelatex"		=> "dev-texlive/texlive-xetex media-fonts/dejavu",
···
 {
 	# Distro-specific hints
 	if ($system_release =~ /Red Hat Enterprise Linux/) {
+		give_redhat_hints;
+		return;
+	}
+	if ($system_release =~ /CentOS/) {
+		give_redhat_hints;
+		return;
+	}
+	if ($system_release =~ /Scientific Linux/) {
+		give_redhat_hints;
+		return;
+	}
+	if ($system_release =~ /Oracle Linux Server/) {
 		give_redhat_hints;
 		return;
 	}
···
 sub check_needs()
 {
 	if ($system_release) {
-		print "Checking if the needed tools for $system_release are available\n";
+		print "Detected OS: $system_release.\n";
 	} else {
-		print "Checking if the needed tools are present\n";
+		print "Unknown OS\n";
+	}
+
+	# RHEL 7.x and clones have Sphinx version 1.1.x and incomplete texlive
+	if (($system_release =~ /Red Hat Enterprise Linux/) ||
+	    ($system_release =~ /CentOS/) ||
+	    ($system_release =~ /Scientific Linux/) ||
+	    ($system_release =~ /Oracle Linux Server/)) {
+		$virtualenv = 1;
+		$pdf = 0;
+
+		printf("NOTE: On this distro, Sphinx and TexLive shipped versions are incompatible\n");
+		printf("with doc build. So, use Sphinx via a Python virtual environment.\n\n");
+		printf("This script can't install a TexLive version that would provide PDF.\n");
 	}
 
 	# Check for needed programs/tools