Global (FreeBSD): fixes build when libc defines memrchr as a macro (#2599)

FreeBSD 16.0-CURRENT's <string.h> now provides C23-style
qualifier-preserving wrappers for the string search functions.  Besides
the prototype, `memrchr` (like `memchr`, `strchr`, `strstr`, `memmem`,
...) is also defined as a function-like macro built on `_Generic`:

    #define memrchr(b, c, n) __qualsel((b), \
        (const void *)(memrchr)((b), (c), (n)), (memrchr)((b), (c), (n)))

`common/memrchr.h` redeclares the function unconditionally.  When
<string.h> has already been included, that declaration is expanded by
the macro and every translation unit fails with:

    src/common/memrchr.h:11:7: error: expected identifier or '('
       11 | void* memrchr(const void* s, int c, size_t n);

A libc that defines `memrchr` as a macro has necessarily declared the
function it wraps, so skip our declaration in that case.  Nothing
changes on platforms where `memrchr` is not a macro, and the fallback
implementation is still selected by `check_function_exists`.

src/common/memrchr.h

@@ -8,7 +8,9 @@ extern "C" {
 
 // `memrchr` is a GNU extension and may not be declared by system headers even when the symbol exists.
 // Declare it unconditionally; the build system provides a fallback implementation when missing.
+#ifndef memrchr
 void* memrchr(const void* s, int c, size_t n);
+#endif
 
 #ifdef __cplusplus
 }