Clean up inconsistent backslash use in paths

Most of the MSVC Perl code uses forward slashes for file paths.  Make
the few places that use backslashes the same.  This also helps running
that code on non-Windows.

src/tools/msvc/build.pl

@@ -12,9 +12,9 @@ use Cwd;
 
 use Mkvcbuild;
 
-chdir('..\..\..') if (-d '..\msvc' && -d '..\..\..\src');
+chdir('../../..') if (-d '../msvc' && -d '../../../src');
 die 'Must run from root or msvc directory'
-  unless (-d 'src\tools\msvc' && -d 'src');
+  unless (-d 'src/tools/msvc' && -d 'src');
 
 # buildenv.pl is for specifying the build environment settings
 # it should contain lines like:

src/tools/msvc/mkvcbuild.pl

@@ -13,9 +13,9 @@ BEGIN { use lib File::Spec->rel2abs(dirname(__FILE__)); }
 
 use Mkvcbuild;
 
-chdir('..\..\..') if (-d '..\msvc' && -d '..\..\..\src');
+chdir('../../..') if (-d '../msvc' && -d '../../../src');
 die 'Must run from root or msvc directory'
-  unless (-d 'src\tools\msvc' && -d 'src');
+  unless (-d 'src/tools/msvc' && -d 'src');
 
 die 'Could not find config_default.pl'
   unless (-f 'src/tools/msvc/config_default.pl');