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

@@ -335,7 +335,8 @@ 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)
-PROVE_FLAGS = --verbose
+# 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/test/perl/README

@@ -14,6 +14,11 @@ src/test/ssl, or should be added to one of the suites for an existing utility.
 Note that all tests and test tools should have perltidy run on them before
 patches are submitted, using perltidy --profile=src/tools/pgindent/perltidyrc
 
+By default, to keep the noise low during runs, we do not set any flags via
+PROVE_FLAGS, but this can be done on the 'make' command line if desired, eg:
+
+make check-world PROVE_FLAGS='--verbose'
+
 Writing tests
 -------------
 

src/tools/msvc/vcregress.pl

@@ -180,7 +180,10 @@ 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;