Fix mistakes in commit d45099425e.

I intended to include a change to the "skip" count in the test
case, but it didn't get folded into the commit. Do that now,
so that non-zlib builds don't break.

The new file bbstreamer_gzip.c needs <unistd.h> to avoid
complaints about dup() not having a prototype, as per buildfarm
returns.

src/bin/pg_basebackup/bbstreamer_gzip.c

@@ -11,6 +11,8 @@
 
 #include "postgres_fe.h"
 
+#include <unistd.h>
+
 #ifdef HAVE_LIBZ
 #include <zlib.h>
 #endif

src/bin/pg_verifybackup/t/009_extract.pl

@@ -36,7 +36,7 @@ for my $tc (@test_configuration)
 	my $method = $tc->{'compression_method'};
 
 	SKIP: {
-		skip "$method compression not supported by this build", 3
+		skip "$method compression not supported by this build", 2
 			if ! $tc->{'enabled'};
 
 		# Take backup with server compression enabled.