Allow MSVC .bat wrappers to be called from anywhere

Instead of using a hardcoded or default path to the perl file the .bat
file is a wrapper for, we use a path that means the file is found in
the same directory as the .bat file.

Patch by Anton Voloshin, slightly tweaked by me.

Backpatch to all live branches

Discussion: https://postgr.es/m/2b7a674b-5fb0-d264-75ef-ecc7a31e54f8@postgrespro.ru

src/tools/msvc/build.bat

@@ -3,4 +3,4 @@ REM src/tools/msvc/build.bat
 REM all the logic for this now belongs in build.pl. This file really
 REM only exists so you don't have to type "perl build.pl"
 REM Resist any temptation to add any logic here.
-@perl build.pl %*
+@perl %~dp0/build.pl %*

src/tools/msvc/install.bat

@@ -3,4 +3,4 @@ REM src/tools/msvc/install.bat
 REM all the logic for this now belongs in install.pl. This file really
 REM only exists so you don't have to type "perl install.pl"
 REM Resist any temptation to add any logic here.
-@perl install.pl %*
+@perl %~dp0/install.pl %*

src/tools/msvc/pgbison.bat

@@ -4,4 +4,4 @@ REM src/tools/msvc/pgbison.bat
 REM all the logic for this now belongs in pgbison.pl. This file really
 REM only exists so you don't have to type "perl src/tools/msvc/pgbison.pl"
 REM Resist any temptation to add any logic here.
-@perl src/tools/msvc/pgbison.pl %*
+@perl %~dp0/pgbison.pl %*

src/tools/msvc/pgflex.bat

@@ -4,4 +4,4 @@ REM src/tools/msvc/pgflex.bat
 REM all the logic for this now belongs in pgflex.pl. This file really
 REM only exists so you don't have to type "perl src/tools/msvc/pgflex.pl"
 REM Resist any temptation to add any logic here.
-@perl src/tools/msvc/pgflex.pl %*
+@perl %~dp0/pgflex.pl %*

src/tools/msvc/vcregress.bat

@@ -3,4 +3,4 @@ REM src/tools/msvc/vcregress.bat
 REM all the logic for this now belongs in vcregress.pl. This file really
 REM only exists so you don't have to type "perl vcregress.pl"
 REM Resist any temptation to add any logic here.
-@perl vcregress.pl %*
+@perl %~dp0/vcregress.pl %*