Use cc(1)'s __has_include() instead of build-system checks

[alejandro-colomar]

All relevant compilers have had __has_include() since immemorial times (GCC since version 5, according to https://en.cppreference.com/w/c/compiler_support). It's now a C23 standard feature.

This reduces the complexity of the build system.

---

Revisions:

v1b

• Rebase

$ git range-diff master..gh/has_include shadow/master..has_include 
1:  f716f558 = 1:  1afc7448 configure.ac, lib/: Use __has_include(<crypt.h>) instead of HAVE_CRYPT_H
2:  95ca17af = 2:  6107b7f8 configure.ac, lib/: Use __has_include(<sys/random.h>) instead of HAVE_SYS_RANDOM_H
3:  3d98527a = 3:  0a32d621 lib/: Use __has_include(<security/pam_misc.h>) instead of HAVE_SECURITY_PAM_MISC_H
4:  8052bd28 = 4:  fe3a23d3 lib/: Use __has_include(<security/openpam.h>) instead of HAVE_SECURITY_OPENPAM_H
5:  114cfa89 = 5:  3fd1baab lib/idmapping.c: Unconditionally include <sys/prctl.h>
6:  4c72c6c3 = 6:  17eef32a configure.ac, lib/: Use __has_include(<sys/capability.h>) instead of HAVE_SYS_CAPABILITY_H
7:  60583eb9 = 7:  c4a55a7a configure.ac: Remove unused AC_CHECK_HEADERS() checks

v2

• Rebase

$ git range-diff --creation-factor=99 master..gh/has_include shadow/master..has_include 
1:  1afc7448 = 1:  25b8fb8f configure.ac, lib/: Use __has_include(<crypt.h>) instead of HAVE_CRYPT_H
2:  6107b7f8 ! 2:  6a9992c2 configure.ac, lib/: Use __has_include(<sys/random.h>) instead of HAVE_SYS_RANDOM_H
    @@ configure.ac: dnl Checks for libraries.
     
      ## lib/csrand.c ##
     @@
    - #include <stdio.h>
    + #include <stdint.h>
      #include <stdlib.h>
      #include <unistd.h>
     -#if HAVE_SYS_RANDOM_H
    @@ lib/csrand.c
     +#if __has_include(<sys/random.h>)
     +# include <sys/random.h>
      #endif
    -+
    + 
      #include "bit.h"
    - #include "defines.h"
    - #include "prototypes.h"
3:  0a32d621 = 3:  6df8270f lib/: Use __has_include(<security/pam_misc.h>) instead of HAVE_SECURITY_PAM_MISC_H
4:  fe3a23d3 = 4:  39255d70 lib/: Use __has_include(<security/openpam.h>) instead of HAVE_SECURITY_OPENPAM_H
5:  3fd1baab ! 5:  00f96fc4 lib/idmapping.c: Unconditionally include <sys/prctl.h>
    @@ lib/idmapping.c
      #include <stdlib.h>
      #include <stdio.h>
      #include <strings.h>
    -+#include <sys/prctl.h>
    - 
    - #include "alloc/calloc.h"
    - #include "alloc/x/xmalloc.h"
    -@@
    - #include "string/sprintf/stpeprintf.h"
    - #include "idmapping.h"
    - #if HAVE_SYS_CAPABILITY_H
    --#include <sys/prctl.h>
    +-#if HAVE_SYS_CAPABILITY_H
    + #include <sys/prctl.h>
    ++#if HAVE_SYS_CAPABILITY_H
      #include <sys/capability.h>
      #endif
    - #include "shadowlog.h"
    + 
6:  17eef32a ! 6:  3ace1b19 configure.ac, lib/: Use __has_include(<sys/capability.h>) instead of HAVE_SYS_CAPABILITY_H
    @@ lib/idmapping.c
      #include <stdio.h>
      #include <strings.h>
      #include <sys/prctl.h>
    -+#if __has_include(<sys/capability.h>)
    -+# include <sys/capability.h>
    -+#endif
    - 
    - #include "alloc/calloc.h"
    - #include "alloc/x/xmalloc.h"
    -@@
    - #include "prototypes.h"
    - #include "string/sprintf/stpeprintf.h"
    - #include "idmapping.h"
     -#if HAVE_SYS_CAPABILITY_H
     -#include <sys/capability.h>
    --#endif
    - #include "shadowlog.h"
    - #include "sizeof.h"
    ++#if __has_include(<sys/capability.h>)
    ++# include <sys/capability.h>
    + #endif
      
    + #include "alloc/calloc.h"
     @@ lib/idmapping.c: get_map_ranges(int ranges, int argc, char **argv)
       */
      #define ULONG_DIGITS (((WIDTHOF(unsigned long) + 9)/10)*3)
7:  c4a55a7a = 7:  2759f057 configure.ac: Remove unused AC_CHECK_HEADERS() checks