Fix the Windows environment check, which cmd could not parse

The messages that report a missing prerequisite were changed from !VAR!
to %VAR% in commit afa0c773a, since the file does not enable delayed
expansion and they printed the variable name rather than the path. That
made them correct and unparseable at the same time: cmd expands %VAR%
when it parses a parenthesised block rather than when it runs it, and
PGADMIN_INNOTOOL_DIR defaults to "C:\Program Files (x86)\Inno Setup 6",
whose closing parenthesis then ended the IF block early. Every Windows
build has failed at "Checking the environment..." since, with "\Inno was
unexpected at this time", whether or not Inno Setup was installed, which
took the nightly snapshot with it.

The four checks move into a :REQUIRE_DIR subroutine that takes the
directory and the hint, and echoes both outside any block, where a
parenthesis is just a character. Enabling delayed expansion would also
have worked, but only by making ! a metacharacter for the whole file.
The other uses of these two paths are all inside double quotes, which
the parser does respect, so they were never affected.

Make.bat

@@ -177,31 +177,16 @@ REM Main build sequence Ends
     ECHO ****************************************************************
 
     ECHO Checking the environment...
-    IF NOT EXIST "%PGADMIN_INNOTOOL_DIR%" (
-        ECHO %PGADMIN_INNOTOOL_DIR% does not exist
-        ECHO Please install InnoTool and set the PGADMIN_INNOTOOL_DIR environment variable.
-        EXIT /B 1
-    )
-
-    IF NOT EXIST "%PGADMIN_VCREDIST_DIR%" (
-        ECHO %PGADMIN_VCREDIST_DIR% does not exist
-        ECHO Please install Microsoft Visual studio and set the PGADMIN_VCREDIST_DIR environment variable.
-        EXIT /B 1
-    )
-
-    IF NOT EXIST "%PGADMIN_KRB5_DIR%" (
-        ECHO %PGADMIN_KRB5_DIR% does not exist.
-        ECHO Please install MIT Kerberos for Windows, from the winpgbuild project or
-        ECHO elsewhere, and set the PGADMIN_KRB5_DIR environment variable.
-        EXIT /B 1
-    )
-
-    IF NOT EXIST "%PGADMIN_POSTGRES_DIR%" (
-        ECHO %PGADMIN_POSTGRES_DIR% does not exist.
-        ECHO Please install PostgreSQL, from the winpgbuild project or elsewhere, and
-        ECHO set the PGADMIN_POSTGRES_DIR environment variable.
-        EXIT /B 1
-    )
+    REM The checks are made by :REQUIRE_DIR rather than inline, because the
+    REM message names the directory that was looked for, and a default such as
+    REM "C:\Program Files (x86)\Inno Setup 6" carries a closing parenthesis.
+    REM Inside a parenthesised block that parenthesis ends the block, as cmd
+    REM expands the variable when it parses the block rather than when it runs it,
+    REM so the block failed to parse whether or not the directory was there.
+    CALL :REQUIRE_DIR "%PGADMIN_INNOTOOL_DIR%" "Please install InnoTool and set the PGADMIN_INNOTOOL_DIR environment variable." || EXIT /B 1
+    CALL :REQUIRE_DIR "%PGADMIN_VCREDIST_DIR%" "Please install Microsoft Visual Studio and set the PGADMIN_VCREDIST_DIR environment variable." || EXIT /B 1
+    CALL :REQUIRE_DIR "%PGADMIN_KRB5_DIR%" "Please install MIT Kerberos for Windows, from the winpgbuild project or elsewhere, and set the PGADMIN_KRB5_DIR environment variable." || EXIT /B 1
+    CALL :REQUIRE_DIR "%PGADMIN_POSTGRES_DIR%" "Please install PostgreSQL, from the winpgbuild project or elsewhere, and set the PGADMIN_POSTGRES_DIR environment variable." || EXIT /B 1
 
     SET "PATH=%PGADMIN_POSTGRES_DIR%\bin;%PATH%"
 
@@ -569,3 +554,16 @@ REM and some of them are optional.
 :CHECK_ROBOCOPY_ERROR
     IF %ERRORLEVEL% GEQ 8 EXIT /B %ERRORLEVEL%
     EXIT /B 0
+
+
+REM Check that a prerequisite directory exists, reporting the path that was
+REM looked for and how to correct it if it does not. The first argument is the
+REM directory and the second the hint; both are echoed here, outside any
+REM parenthesised block, so a parenthesis in either is just a character.
+:REQUIRE_DIR
+    IF EXIST "%~1" EXIT /B 0
+
+    ECHO %~1 does not exist.
+    ECHO %~2
+
+    EXIT /B 1