fixed optxxx var undefined in src/webpng.c under MSVC

[willson-chen]

unistd.h can not included by src/webpng.c under MSVC. So optind opterr and related var are unavaible.

[willson-chen]

@vapier hi, could you please spare some time to offer some suggestion? I want improve the build under windows platform.

[willson-chen]

Hi @vapier @cmb69, could you please spare some time to review this PR.

I have test this PR on win10 with cmake and it works.

But getopt is only used in webpng.c, which is not the lib source file, but a tool source file. Beside it, there are gdtopng.c, pngtogd.c, gdparttopng, gd2topng, pngtogd2, annotate, gd2copyal, gd2togif, giftogd2 in tool source file list. I don't know if someone still use these tools, but I think it is better to move them to a new folder named tools. The build failure of these tool source files block adding windows platform to CI.

What do you think? :)

[cmb69]

This looks okay to me. I wonder. though, if it makes sense to maintain these tools; they actually look more like examples to me, and their names are somewhat arbitrary.

[willson-chen]

they actually look more like examples to me

Take gd2topng.cfor example. It generates a executable program, which is used to transform image from gd2 to png formats. It can be treated as a tools, or an example to show the transformation.

I strongly recommand to move them to a new folder, make srcdir only contain the source files used to generate the library file. What do you think we should deal with them?

[stima]

I'm curious if there is necessity in having cpp guard for c library header

[stima]

Header is good way to declare functions, but I haven't seen headers in this library and this functionality is used only in one place.