Create INSTALL file using Pandoc

Replace using lynx with using pandoc.  Pandoc creates better looking
output and it avoids the delicate locale/encoding issues of lynx because
it always uses UTF-8 for both input and output.

Note: requires Pandoc >=1.13

Discussion: https://www.postgresql.org/message-id/flat/dcfaa74d-8037-bb32-f9e0-3fea7ccf4551@2ndquadrant.com/
Reviewed-by: Mi Tar <mmitar@gmail.com>

doc/src/sgml/Makefile

@@ -100,18 +100,10 @@ errcodes-table.sgml: $(top_srcdir)/src/backend/utils/errcodes.txt generate-errco
 ##
 
 ICONV = iconv
-LYNX = lynx
-
-# The documentation may contain non-ASCII characters (mostly for
-# contributor names), which lynx converts to the encoding determined
-# by the current locale.  To get text output that is deterministic and
-# easily readable by everyone, we make lynx produce LATIN1 and then
-# convert that to ASCII with transliteration for the non-ASCII characters.
-# Official releases were historically built on FreeBSD, which has limited
-# locale support and is very picky about locale name spelling.  The
-# below has been finely tuned to run on FreeBSD and Linux/glibc.
+PANDOC = pandoc
+
 INSTALL: % : %.html
-	$(PERL) -p -e 's,<h(1|2) class="title",<h\1 align=center,g' $< | LC_ALL=en_US.ISO8859-1 $(LYNX) -force_html -dump -nolist -stdin | $(ICONV) -f latin1 -t us-ascii//TRANSLIT > $@
+	$(PANDOC) $< -t plain | $(ICONV) -f utf8 -t us-ascii//TRANSLIT > $@
 
 INSTALL.html: %.html : stylesheet-text.xsl %.xml
 	$(XMLLINT) --noout --valid $*.xml

doc/src/sgml/docguide.sgml

@@ -358,7 +358,9 @@ ADDITIONAL_FLAGS='-Xmx1500m'
     corresponds to <xref linkend="installation"/>, with some minor
     changes to account for the different context.  To recreate the
     file, change to the directory <filename>doc/src/sgml</filename>
-    and enter <userinput>make INSTALL</userinput>.
+    and enter <userinput>make INSTALL</userinput>.  Building text output
+    requires <productname>Pandoc</productname> version 1.13 or newer as an
+    additional build tool.
    </para>
 
    <para>