utils.c: include <signal.h> for siginfo_t

[aitap]

POSIX says:

The <signal.h> header shall define the siginfo_t type as a structure

So <sys/wait.h> is not enough to see the definition (not just a forward declaration) of siginfo_t.

Fixes: #7516

Tested manually on FreeBSD:

Details

$ R CMD check data.table_1.18.99.tar.gz --no-manual

* using R version 4.5.1 (2025-06-13)                                                   
* using platform: amd64-portbld-freebsd15.0                                            
* R was compiled by                                                                                                                                                            
    FreeBSD clang version 19.1.7 (https://github.com/llvm/llvm-project.git llvmorg-19.1.7-0-gcd708029e0b2)
    GNU Fortran (FreeBSD Ports Collection) 13.3.0                                      
* running under: FreeBSD freebsd 15.0-RELEASE FreeBSD 15.0-RELEASE releng/15.0-n280995-7aedc8de6446 GENERIC amd64
* checking whether package ‘data.table’ can be installed ... OK                        
* used C compiler: ‘FreeBSD clang version 19.1.7 (https://github.com/llvm/llvm-project.git llvmorg-19.1.7-0-gcd708029e0b2)’                                                    
* checking compiled code ... NOTE                                                      
File ‘data.table/libs/data_table.so’:   
  Found non-API calls to R: ‘IS_GROWABLE’, ‘LEVELS’, ‘SETLENGTH’,
    ‘SET_GROWABLE_BIT’, ‘SET_TRUELENGTH’, ‘TRUELENGTH’, ‘XTRUELENGTH’
Status: 1 NOTE                                                                         

...and OpenBSD:

Details

$ LANG=C.UTF-8 R CMD check data.table_1.18.99.tar.gz --no-manual

* R was compiled by                                                                    
    egcc (GCC) 8.4.0                                                                   
    GNU Fortran (GCC) 8.4.0                                                            
* running under: OpenBSD openbsd.my.domain 7.8 GENERIC#54 amd64                        

* checking whether package ‘data.table’ can be installed ... OK
* used C compiler: ‘OpenBSD clang version 19.1.7’

Status: OK

Also tested on GNU/Linux with PKG_CFLAGS=--std=c99.

@jszhao, could you please pull from this branch (posix_siginfo_t) and run R CMD build, then install the resulting tarball?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.97%. Comparing base (a29c209) to head (baaf8ca).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7517   +/-   ##
=======================================
  Coverage   98.97%   98.97%           
=======================================
  Files          87       87           
  Lines       16740    16740           
=======================================
  Hits        16568    16568           
  Misses        172      172           

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[MichaelChirico]

i guess a better description is "failure in some strict environments, e.g. FreeBSD, due..." WDYT?

Maybe worth mentioning the POSIX compliance aspect too?

[github-actions]

• HEAD=posix_siginfo_t stopped early for DT[by,verbose=TRUE] improved in #6296
  

Generated via commit baaf8ca

Download link for the artifact containing the test results: ↓ atime-results.zip

Task	Duration
R setup and installing dependencies	2 minutes and 46 seconds
Installing different package versions	22 seconds
Running and plotting the test cases	4 minutes and 5 seconds

[HughParsonage]

# if !defined(_POSIX_C_SOURCE) || _POSIX_C_SOURCE < 200809L
#  undef _POSIX_C_SOURCE
#  define _POSIX_C_SOURCE 200809L
# endif

for robustness