Allow MSVC to build with Tcl 8.6.

Commit eaba54c added support for Tcl 8.6 for configure-supported
platforms after verifying that pltcl works without further changes, but
the MSVC tooling wasn't updated accordingly.  Update MSVC to match,
restructuring the code to avoid duplicating the logic for every Tcl
version supported.

Backpatch to all live branches, like eaba54c.  In 9.4 and previous,
change the patch to use backslashes rather than forward, as in the rest
of the file.

Reported by Paresh More, who also tested the patch I provided.
Discussion: https://postgr.es/m/CAAgiCNGVw3ssBtSi3ZNstrz5k00ax=UV+_ZEHUeW_LMSGL2sew@mail.gmail.com

Author: alvherre

src/tools/msvc/Mkvcbuild.pm

@@ -194,20 +194,24 @@ sub mkvcbuild
 
 	if ($solution->{options}->{tcl})
 	{
+		my $found = 0;
 		my $pltcl =
 		  $solution->AddProject('pltcl', 'dll', 'PLs', 'src/pl/tcl');
 		$pltcl->AddIncludeDir($solution->{options}->{tcl} . '/include');
 		$pltcl->AddReference($postgres);
-		if (-e $solution->{options}->{tcl} . '/lib/tcl85.lib')
-		{
-			$pltcl->AddLibrary(
-				$solution->{options}->{tcl} . '/lib/tcl85.lib');
-		}
-		else
+
+		for my $tclver (qw(86t 85 84))
 		{
-			$pltcl->AddLibrary(
-				$solution->{options}->{tcl} . '/lib/tcl84.lib');
+			my $tcllib = $solution->{options}->{tcl} . "/lib/tcl$tclver.lib";
+			if (-e $tcllib)
+			{
+				$pltcl->AddLibrary($tcllib);
+				$found = 1;
+				last;
+			}
 		}
+		die "Unable to find $solution->{options}->{tcl}/lib/tcl<version>.lib"
+			unless $found;
 	}
 
 	$libpq = $solution->AddProject('libpq', 'dll', 'interfaces',