I'm not sure if this is an issue in the configure test that postgres is using. If so, I'll report it over there or to autoconf as appropriate.

(I think pg is doing something wrong here anyway as it's using CFLAGS it tests for itself for all of its configure tests.)

Anyway, I originally reported this downstream in Gentoo at https://bugs.gentoo.org/923804.

pg fails to build like:
```
pg_collation.c:46:1: error: conflicting types for ‘CollationCreate’; have ‘Oid(const char *, Oid,  Oid,  char,  _Bool,  int32,  const char *, const char *, const char *, const char *, const char *, _Bool,  _Bool)’ {aka ‘unsigned int(const char *, unsigned int,  unsigned int,  char,  _Bool,  int,  const char *, const char *, const char *, const char *, const char *, _Bool,  _Bool)’}
   46 | CollationCreate(const char *collname, Oid collnamespace,
      | ^~~~~~~~~~~~~~~
In file included from pg_collation.c:25:
../../../src/include/catalog/pg_collation.h:88:17: note: previous declaration of ‘CollationCreate’ with type ‘Oid(const char *, Oid,  Oid,  char,  bool,  int32,  const char *, const char *, const char *, const char *, const char *, bool,  bool)’ {aka ‘unsigned int(const char *, unsigned int,  unsigned int,  char,  unsigned char,  int,  const char *, const char *, const char *, const char *, const char *, unsigned char,  unsigned char)’}
   88 | extern Oid      CollationCreate(const char *collname, Oid collnamespace,
      |                 ^~~~~~~~~~~~~~~
pg_collation.c: In function ‘CollationCreate’:
pg_collation.c:207:48: error: passing argument 3 of ‘heap_form_tuple’ from incompatible pointer type [-Wincompatible-pointer-types]
  207 |         tup = heap_form_tuple(tupDesc, values, nulls);
      |                                                ^~~~~
      |                                                |
      |                                                _Bool *
In file included from pg_collation.c:18:
../../../src/include/access/htup_details.h:715:87: note: expected ‘bool *’ {aka ‘unsigned char *’} but argument is of type ‘_Bool *’
  715 |                                                                  Datum *values, bool *isnull);
```

This turns out to be because of a configure test getting confused:
```

#include <stdbool.h>
#ifndef bool
"error: bool is not defined"
#endif
#ifndef false
    "error: false is not defined"
#endif
#if false
"error: false is not 0"
#endif
#ifndef true
    "error: true is not defined"
#endif
#if true != 1
    "error: true is not 1"
#endif
#ifndef __bool_true_false_are_defined
    "error: __bool_true_false_are_defined is not defined"
#endif

    struct s {
    _Bool s : 1;
    _Bool t;
} s;

char a[true == 1 ? 1 : -1];
char b[false == 0 ? 1 : -1];
char c[__bool_true_false_are_defined == 1 ? 1 : -1];
char d[(bool)0.5 == true ? 1 : -1];
/* See body of main program for 'e'.  */
char f[(_Bool)0.0 == false ? 1 : -1];
char g[true];
char h[sizeof(_Bool)];
char i[sizeof s.t];
enum { j = false, k = true, l = false * true, m = true * 256 };
/* The following fails for
HP aC++/ANSI C B3910B A.05.55 [Dec 04 2003]. */
_Bool n[m];
char o[sizeof n == m * sizeof n[0] ? 1 : -1];
char p[-1 - (_Bool)0 < 0 && -1 - (bool)0 < 0 ? 1 : -1];
/* Catch a bug in an HP-UX C compiler.  See
http://gcc.gnu.org/ml/gcc-patches/2003-12/msg02303.html
http://lists.gnu.org/archive/html/bug-coreutils/2005-11/msg00161.html
*/
_Bool q = true;
_Bool *pq = &q;

int main() {
    bool e = &s;
    *pq |= q;
    *pq |= !q;
    /* Refer to every declared value, to avoid compiler optimizations.  */
    return (!a + !b + !c + !d + !e + !f + !g + !h + !i + !!j + !k + !!l + !m +
            !n + !o + !p + !q + !pq);
    return 0;
}
```

With -O2 -Werror=vla, it recently started emitting:
```
<source>:30:1: warning: variably modified 'd' at file scope
   30 | char d[(bool)0.5 == true ? 1 : -1];
      | ^~~~
<source>:30:1: error: ISO C90 forbids array 'd' whose size cannot be evaluated [-Werror=vla]
<source>:32:1: warning: variably modified 'f' at file scope
   32 | char f[(_Bool)0.0 == false ? 1 : -1];
      | ^~~~
<source>:32:1: error: ISO C90 forbids array 'f' whose size cannot be evaluated [-Werror=vla]
cc1: some warnings being treated as errors
Compiler returned: 1
```

Is this intentional or not? GCC 13 doesn't error, nor does Clang 17.