Take PROVE_FLAGS from the command line but not the environment

This reverts commit 56b6ef89 and instead
makes vcregress.pl parse out PROVE_FLAGS from a command line argument
when doing a TAP test, thus making it consistent with the makefile
treatment.

Discussion: https://postgr.es/m/c26a7416-2fb9-34ab-7991-618c922f896e%402ndquadrant.com

Backpatch to 9.4 like previous patch.

src/Makefile.global.in

@@ -340,7 +340,7 @@ PROVE = @PROVE@
 # extra perl modules in their own directory.
 PG_PROVE_FLAGS = -I $(top_srcdir)/src/test/perl/ -I $(srcdir)
 # User-supplied prove flags such as --verbose can be provided in PROVE_FLAGS.
-
+PROVE_FLAGS =
 
 # prepend to path if already set, else just set it
 define add_to_path

src/tools/msvc/vcregress.pl

@@ -178,12 +178,18 @@ sub tap_check
 	die "Tap tests not enabled in configuration"
 	  unless $config->{tap_tests};
 
+	my @flags;
+	foreach my $arg (0 .. scalar(@_))
+	{
+		next unless $_[$arg] =~ /^PROVE_FLAGS=(.*)/;
+		@flags = split(/\s+/, $1);
+		splice(@_,$arg,1);
+		last;
+	}
+
 	my $dir = shift;
 	chdir $dir;
 
-	my @flags;
-	@flags = split(/\s+/, $ENV{PROVE_FLAGS}) if exists $ENV{PROVE_FLAGS};
-
 	my @args = ("prove", @flags, "t/*.pl");
 
 	# adjust the environment for just this test