Build fails with libpng 1.4.x

[vog]

On a system with libpng 1.4.x, a "configure --with-libpng=sys" fails with:

---

checking for png.h > 0.90...
checking for png.h... yes
checking for png_check_sig in -lpng... no
configure: error: system png library not found or too old! Use --with-libpng=builtin to use built-in version

---

It checks for the deprecated function png_check_sig(). Since that function isn't used by wxWidgets anyway, checking instead for png_sig_cmp() solves the problem.

The attached patch does exactly that. Note that it changes just configure.ac, so the configure script needs to be recreated after applying the patch.

More information:
 http://www.libpng.org/pub/png/src/libpng-1.2.x-to-1.4.x-summary.txt

[vog]

Check for png_sig_cmp() instead of png_check_sig()

[vadz]

Thank you, will apply in a moment.

It would also be nice to upgrade the libpng version included in our tree, if anybody wants to do it, it would be very welcome.

P.S. I assumed you're the same "vog" as at SF so I used "Volker Grabsch" to credit you in docs/changes.txt, please let me know if I was wrong.

[VZ]

(In [63137]) Check for a function existing in all libpng releases in configure.

We tested for png_check_sig() which was deprecated and is not available in the
latest libpng 1.4 any more. Just use another, not deprecated and not new,
function for the test.

See #11625.

[vadz]

Also applied to 2.8 in r63138 (which unfortunately got a completely wrong commit message...).

[vog]

Replying to vadz:

P.S. I assumed you're the same "vog" as at SF so I used "Volker Grabsch" to credit you in docs/changes.txt, please let me know if I was wrong.

Yes, I'm the same "vog". Thanks for the credit.