Fix perl searchpath for modern perl for MSVC tools

Modern versions of perl no longer include the current directory in the
perl searchpath, as it's insecure. Instead of adding the current
directory, we get around the problem by adding the directory where the
script lives.

Problem noted by Victor Wagner.

Solution adapted from buildfarm client code.

Backpatch to all live versions.

src/tools/msvc/install.pl

@@ -6,6 +6,10 @@
 use strict;
 use warnings;
 
+use File::Basename;
+use File::Spec;
+BEGIN  { use lib File::Spec->rel2abs(dirname(__FILE__)); }
+
 use Install qw(Install);
 
 # buildenv.pl is for specifying the build environment settings

src/tools/msvc/mkvcbuild.pl

@@ -7,6 +7,10 @@
 use strict;
 use warnings;
 
+use File::Basename;
+use File::Spec;
+BEGIN  { use lib File::Spec->rel2abs(dirname(__FILE__)); }
+
 use Mkvcbuild;
 
 chdir('..\..\..') if (-d '..\msvc' && -d '..\..\..\src');

src/tools/msvc/vcregress.pl

@@ -11,6 +11,8 @@ use File::Basename;
 use File::Copy;
 use File::Find ();
 use File::Path qw(rmtree);
+use File::Spec;
+BEGIN  { use lib File::Spec->rel2abs(dirname(__FILE__)); }
 
 use Install qw(Install);