Win32: MinGW GCC compatibility.

Several warnings silenced, notably (ngx_socket_t) -1 is now checked
on socket operations instead of -1, as ngx_socket_t is unsigned on win32
and gcc complains on comparison.

With this patch, it's now possible to compile nginx using mingw gcc,
with options we normally compile on win32.
diff --git a/auto/lib/openssl/conf b/auto/lib/openssl/conf
index 528ee17..a65815f 100644
--- a/auto/lib/openssl/conf
+++ b/auto/lib/openssl/conf
@@ -33,6 +33,10 @@
             CORE_LIBS="$CORE_LIBS $OPENSSL/.openssl/lib/libssl.a"
             CORE_LIBS="$CORE_LIBS $OPENSSL/.openssl/lib/libcrypto.a"
             CORE_LIBS="$CORE_LIBS $NGX_LIBDL"
+
+            if [ "$NGX_PLATFORM" = win32 ]; then
+                CORE_LIBS="$CORE_LIBS -lgdi32 -lcrypt32 -lws2_32"
+            fi
         ;;
     esac