#144 giflib-5.2.1 cannot be build on windows and other platforms using c89

[mrserb]

I have reported both issues before, but it looks like they are not completely fixed.

For windows it is necessary to remove #include <unistd.h> from the gif_hash.h
see bug description: https://sourceforge.net/p/giflib/bugs/129/</unistd.h>

And for c89 compileres the <stdbool.h> should not be used, see my comment here:
https://sourceforge.net/p/giflib/bugs/130/</stdbool.h>

[Eric S. Raymond]

I've added a guard around the unistd.h header.

As for C89, it's 2024 and I'm not going to abandon good modern C practice for compilers 35 years out of date. One has to draw the line somewhere, My answer to anyone actually affected by this is "fix your toolchain."