I would recommend rolling your own converter.

First off, you should target your efforts towards the '.vcproj' files - as they are holding the actual files in the project. Refer to Microsoft for the full XML schema. (Hey, they actually documented this!).

Then use XML::Simple or whatever our preference is to parse, rinse, stir and output to a nmake/gmake/whatnot file.

The '.sln' file is not XML, but a pretty straight-forward text-file, most notably linking to the '.vcproj' files.

I haven't converted in this direction, but rather generated .vcproj files from other input.

Following code should get you started (assumes '.sln' file(s) as args):

use strict;
use warnings;
use XML::Simple;
use Data::Dumper;

my(%projs);
while(<>) {
  # Extract project files linked into solution
  if(/^Project\(.+\)\s+=\s+\"([^"]+)\",\s+\"([^"]+)\"/) {
    $projs{$1} = $2;   # Got one!
  }
}

# Process the project files - XML format
my $xs = new XML::Simple();
for my $p (keys(%projs)) {
  my $file = $projs{$p};
  print "Parsing $p, file $file\n";
  my $ref = $xs->XMLin($file);
  # Print parsed XML - or output in another format?
  print Dumper($ref);
}
[download]

Cheers,

---Lars

Update: Added code snippet