fragment: Mingw needs to use POSIX handling for `\\`

Otherwise it won't correctly escape things like spaces, and can print
out the `\` literal escaping nothing, which causes most shell lexers to
choke.

Fixes: #238

libpkgconf/fragment.c

@@ -438,7 +438,7 @@ fragment_quote(const pkgconf_fragment_t *frag)
 		    (*src > ':' && *src < '=') ||
 		    (*src > '=' && *src < '@') ||
 		    (*src > 'Z' && *src < '\\') ||
-#ifndef _WIN32
+#if !defined(_WIN32) && !defined(__MINGW32__)
 		    (*src == '\\') ||
 #endif
 		    (*src > '\\' && *src < '^') ||