Honor PROVE_FLAGS environment setting

On MSVC builds and on back branches that means removing the hardcoded
--verbose setting. On master for Unix that means removing the empty
setting in the global Makefile so that the value can be acquired from
the environment as well as from the make arguments.

Backpatch to 9.4 where we introduced TAP tests

src/Makefile.global.in

@@ -339,8 +339,8 @@ PROVE = @PROVE@
 # There are common routines in src/test/perl, and some test suites have
 # extra perl modules in their own directory.
 PG_PROVE_FLAGS = -I $(top_srcdir)/src/test/perl/ -I $(srcdir)
-# For more info, add to your make line: PROVE_FLAGS='--verbose'
-PROVE_FLAGS =
+# User-supplied prove flags such as --verbose can be provided in PROVE_FLAGS.
+
 
 # prepend to path if already set, else just set it
 define add_to_path

src/tools/msvc/vcregress.pl

@@ -181,7 +181,10 @@ sub tap_check
 	my $dir = shift;
 	chdir $dir;
 
-	my @args = ("prove", "--verbose", "t/*.pl");
+	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
 	local %ENV = %ENV;