Make TAP test suites to work, when @INC does not contain current dir.

Recent Perl and/or new Linux distributions are starting to remove "." from
the @INC list by default. That breaks pg_rewind and ssl test suites, which
use helper perl modules that reside in the same directory. To fix, add the
current source directory explicitly to prove's include dir.

The vcregress.pl script probably also needs something like this, but I
wasn't able to remove '.' from @INC on Windows to test this, and don't want
to try doing that blindly.

Discussion: <20160908204529.flg6nivjuwp5vaoy@alap3.anarazel.de>

src/Makefile.global.in

@@ -332,7 +332,9 @@ endif
 endif
 
 PROVE = @PROVE@
-PG_PROVE_FLAGS = -I $(top_srcdir)/src/test/perl/
+# 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
 
 # prepend to path if already set, else just set it