contrib/buildsystems: handle options starting with a slash

With the recent changes to allow building with MSVC=1, we now pass the
/OPT:REF option to the compiler. This confuses the parser that wants to
turn the output of a dry run into project definitions for QMake and Visual
Studio:

	Unhandled link option @ line 213: /OPT:REF at [...]

Let's just extend the code that passes through options that start with a
dash, so that it passes through options that start with a slash, too.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/buildsystems/engine.pl

@@ -351,7 +351,7 @@ sub handleLinkLine
             push(@libs, "expat.lib");
         } elsif ("$part" eq "-liconv") {
             push(@libs, "libiconv.lib");
-        } elsif ($part =~ /^-/) {
+        } elsif ($part =~ /^[-\/]/) {
             push(@lflags, $part);
         } elsif ($part =~ /\.(a|lib)$/) {
             $part =~ s/\.a$/.lib/;