Improve perl script in MSVC to build binaries

This commit includes two improvements for build.pl in src/tools/msvc/:
- Fix two warnings related to $ARGV[0] being uninitialized, something
that happens when calling build.pl (or build.bat) without arguments to
compile all the components with a release quality.
- If calling the script with more than two arguments, exit immediately
and show a new help output.  build.pl was not failing in this case
before this commit, and the extra arguments were just ignored, but the
new behavior helps in understanding how to run the script without
looking at the documentation for the Windows builds.

Reported-by: Ranier Vilela
Author: Michael Paquier
Reviewed-by: Juan José Santamaría Flecha, David Zhang
Discussion: https://postgr.es/m/CAEudQAo38dfR_0Ugt2OHy4mq-6Hz93XPSBAGEUV67RhKdgp8Zg@mail.gmail.com

src/tools/msvc/build.pl

 # -*-perl-*- hey - emacs - this is a perl file
-
+#
+# Script that provides 'make' functionality for msvc builds.
+#
 # src/tools/msvc/build.pl
-
+#
 use strict;
 use warnings;
 
@@ -12,10 +14,22 @@ use Cwd;
 
 use Mkvcbuild;
 
+sub usage
+{
+	die(    "Usage: build.pl [ [ <configuration> ] <component> ]\n"
+		  . "Options are case-insensitive.\n"
+		  . "  configuration: Release | Debug.  This sets the configuration\n"
+		  . "    to build.  Default is Release.\n"
+		  . "  component: name of component to build.  An empty value means\n"
+		  . "    to build all components.\n");
+}
+
 chdir('../../..') if (-d '../msvc' && -d '../../../src');
 die 'Must run from root or msvc directory'
   unless (-d 'src/tools/msvc' && -d 'src');
 
+usage() unless scalar(@ARGV) <= 2;
+
 # buildenv.pl is for specifying the build environment settings
 # it should contain lines like:
 # $ENV{PATH} = "c:/path/to/bison/bin;$ENV{PATH}";
@@ -37,17 +51,20 @@ do "./src/tools/msvc/config.pl" if (-f "src/tools/msvc/config.pl");
 my $vcver = Mkvcbuild::mkvcbuild($config);
 
 # check what sort of build we are doing
-
 my $bconf     = $ENV{CONFIG}   || "Release";
 my $msbflags  = $ENV{MSBFLAGS} || "";
 my $buildwhat = $ARGV[1]       || "";
-if (uc($ARGV[0]) eq 'DEBUG')
-{
-	$bconf = "Debug";
-}
-elsif (uc($ARGV[0]) ne "RELEASE")
+
+if (defined($ARGV[0]))
 {
-	$buildwhat = $ARGV[0] || "";
+	if (uc($ARGV[0]) eq 'DEBUG')
+	{
+		$bconf = "Debug";
+	}
+	elsif (uc($ARGV[0]) ne "RELEASE")
+	{
+		$buildwhat = $ARGV[0] || "";
+	}
 }
 
 # ... and do it