Allow running src/tools/msvc/mkvcbuild.pl under not Windows

This to allow verifying the MSVC build file generation without having
to have Windows.

To do this, we avoid Windows-specific Perl modules and don't run the
"cl" compiler or "nmake".  The resulting build files won't actually be
completely correct, but it's useful enough.
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Reviewed-by: Julien Rouhaud <rjuju123@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/d73b2c7b-f081-8357-8422-7564d55f1aac%402ndquadrant.com

src/tools/msvc/Mkvcbuild.pm

@@ -6,7 +6,7 @@ package Mkvcbuild;
 # src/tools/msvc/Mkvcbuild.pm
 #
 use Carp;
-use Win32;
+use if ($^O eq "MSWin32"), 'Win32';
 use strict;
 use warnings;
 use Project;
@@ -648,9 +648,11 @@ sub mkvcbuild
 					# 'Can't spawn "conftest.exe"'; suppress that.
 					no warnings;
 
+					no strict 'subs';
+
 					# Disable error dialog boxes like we do in the postmaster.
 					# Here, we run code that triggers relevant errors.
-					use Win32API::File qw(SetErrorMode :SEM_);
+					use if ($^O eq "MSWin32"), 'Win32API::File', qw(SetErrorMode :SEM_);
 					my $oldmode = SetErrorMode(
 						SEM_FAILCRITICALERRORS | SEM_NOGPFAULTERRORBOX);
 					system(".\\$exe");

src/tools/msvc/Project.pm

@@ -22,7 +22,7 @@ sub _new
 	my $self = {
 		name                  => $name,
 		type                  => $type,
-		guid                  => Win32::GuidGen(),
+		guid                  => $^O eq "MSWin32" ? Win32::GuidGen() : 'FAKE',
 		files                 => {},
 		references            => [],
 		libraries             => [],

src/tools/msvc/Solution.pm

@@ -60,10 +60,17 @@ sub DeterminePlatform
 {
 	my $self = shift;
 
+	if ($^O eq "MSWin32")
+	{
 		# Examine CL help output to determine if we are in 32 or 64-bit mode.
 		my $output = `cl /? 2>&1`;
 		$? >> 8 == 0 or die "cl command not found";
 		$self->{platform} = ($output =~ /^\/favor:<.+AMD64/m) ? 'x64' : 'Win32';
+	}
+	else
+	{
+		$self->{platform} = 'FAKE';
+	}
 	print "Detected hardware platform: $self->{platform}\n";
 	return;
 }
@@ -1061,7 +1068,7 @@ EOF
 		}
 		if ($fld ne "")
 		{
-			$flduid{$fld} = Win32::GuidGen();
+			$flduid{$fld} = $^O eq "MSWin32" ? Win32::GuidGen() : 'FAKE';
 			print $sln <<EOF;
 Project("{2150E333-8FDC-42A3-9474-1A3956D46DE8}") = "$fld", "$fld", "$flduid{$fld}"
 EndProject

src/tools/msvc/VSObjectFactory.pm

@@ -111,7 +111,8 @@ sub CreateProject
 
 sub DetermineVisualStudioVersion
 {
-
+	if ($^O eq "MSWin32")
+	{
 		# To determine version of Visual Studio we use nmake as it has
 		# existed for a long time and still exists in current Visual
 		# Studio versions.
@@ -126,6 +127,12 @@ sub DetermineVisualStudioVersion
 
 		croak
 		  "Unable to determine Visual Studio version: The nmake version could not be determined.";
+	}
+	else
+	{
+		# fake version
+		return '16.00';
+	}
 }
 
 sub _GetVisualStudioVersion