Doc: specify that the minimum supported version of Perl is 5.8.3.

Previously the docs just said "5.8 or later".  Experimentation shows
that while you can build on Unix from a git checkout with 5.8.0,
compiling recent PL/Perl requires at least 5.8.1, and you won't be
able to run the TAP tests with less than 5.8.3 because that's when
they added "prove".  (I do not have any information on just what the
MSVC build scripts require.)

Since all these versions are quite ancient, let's not split hairs
in the docs, but just say that 5.8.3 is the minimum requirement.

Discussion: https://postgr.es/m/16894.1501392088@sss.pgh.pa.us

doc/src/sgml/install-windows.sgml

@@ -180,7 +180,8 @@ $ENV{MSBFLAGS}="/m";
       ActiveState Perl is required to run the build generation scripts. MinGW
       or Cygwin Perl will not work. It must also be present in the PATH.
       Binaries can be downloaded from
-      <ulink url="http://www.activestate.com"></> (Note: version 5.8 or later is required,
+      <ulink url="http://www.activestate.com"></>
+      (Note: version 5.8.3 or later is required,
       the free Standard Distribution is sufficient).
      </para></listitem>
     </varlistentry>

doc/src/sgml/installation.sgml

@@ -163,6 +163,7 @@ su - postgres
       <application>PL/Perl</application> you need a full
       <productname>Perl</productname> installation, including the
       <filename>libperl</filename> library and the header files.
+      The minimum required version is <productname>Perl</productname> 5.8.3.
       Since <application>PL/Perl</application> will be a shared
       library, the <indexterm><primary>libperl</primary></indexterm>
       <filename>libperl</filename> library must be a shared library
@@ -309,7 +310,7 @@ su - postgres
        <primary>perl</primary>
       </indexterm>
 
-      <application>Perl</> 5.8 or later is needed to build from a Git checkout,
+      <application>Perl</> 5.8.3 or later is needed to build from a Git checkout,
       or if you changed the input files for any of the build steps that
       use Perl scripts.  If building on Windows you will need
       <application>Perl</> in any case.  <application>Perl</application> is