posix: add missing HAVE_ALLOCA_H definition, fixes #272

[sbraz]

Hi,
Without this, the build fails with:

/var/tmp/portage/net-nntp/nzbget-24.1/work/nzbget-24.1/lib/regex/regex.c: In function ‘set_regs’:
/var/tmp/portage/net-nntp/nzbget-24.1/work/nzbget-24.1/lib/regex/regex.c:7701:39: error: implicit declaration of function ‘alloca’; did you mean ‘calloc’? [-Wimplicit-function-declaration]                        7701 |         prev_idx_match = (regmatch_t*)alloca(nmatch * sizeof(regmatch_t));
      |                                       ^~~~~~
      |                                       calloc

Description

This fixes #272.

Lib changes

None.

Testing

This fixes the build on my machine.

[dnzbk]

@sbraz

Thank you but move your changes after this line, please and include(CheckIncludeFile) won't be needed.
regex was vendored in this commit due to an issue with building the application on Windows and I think it wasn't a good solution. I'll try to remove it later.

[sbraz]

Is this better now? I don't really understand how the other check_include_file calls work, they set a HAVE_* variable but never pass it to the compiler?

[dnzbk]

Is this better now?

Yes, better, thanks.

I don't really understand how the other check_include_file calls work, they set a HAVE_* variable but never pass it to the compiler?

For preprocessor definitions we use check_include_file and a file template.
Then the CMake configure_file command does the trick.

If you put HAVE_ALLOCA_H to config.h.in in the same way as the other definitions that would be the best solution.

[sbraz]

Thanks, this makes more sense to me now :) I still don't really see when #cmakedefine HAVE_ALLOCA_H @HAVE_ALLOCA_H@ is expanded but it's no big deal. I think the latest commit follows the existing style as requested.