Update docs on building for Windows to accomodate current reality.

Document how to build 64 bit Windows binaries using the MinGW64 tool set.
Remove recommendation against using Mingw as a build platform.
Be more specific about when Cygwin is useful and when it's not,  in
particular note its usefulness for running psql, and
add a note about building on Cygwin in non-C locales.

Per recent discussions.

doc/src/sgml/installation.sgml

@@ -2312,7 +2312,7 @@ createlang: language installation failed: ERROR:  could not load library "/opt/d
     PostgreSQL can be built using Cygwin, a Linux-like environment for
     Windows, but that method is inferior to the native Windows build
     <![%standalone-ignore[(see <xref linkend="install-windows">)]]> and
-    is no longer recommended.
+    running a server under Cygwin is no longer recommended.
    </para>
 
    <para>
@@ -2371,6 +2371,16 @@ createlang: language installation failed: ERROR:  could not load library "/opt/d
       </para>
      </listitem>
 
+     <listitem>
+      <para>
+        Building might fail on some systems where a locale other than
+        C is in use. To fix this, set the locale to C by doing
+        <command>export LANG=C.utf8</command> before building, and then
+        setting it back to the previous setting, after you have installed
+        PostgreSQL.
+      </para>
+     </listitem>
+
      <listitem>
       <para>
        The parallel regression tests (<literal>make check</literal>)
@@ -2580,6 +2590,14 @@ cc-1020 cc: ERROR File = pqcomm.c, Line = 427
     creating the binaries.
    </para>
 
+   <para>
+     To build 64 bit binaries using MinGW, install the 64 bit tool set
+     from <ulink url="http://www.mingw64.org/"></ulink>, put its bin
+     directory in the <envar>PATH</envar>, and run 
+     <command>configure</command> with the  
+     <command>--host=x86_64-w64-mingw</command> option.
+   </para>
+
    <para>
     After you have everything installed, it is suggested that you
     run <application>psql</application>