Build with clang, and the core and store on macOS, again

Apple clang 21 (an M3) and clang 19 refused what gcc let pass (research
029):
- PURE_FIELD_REST was an enum of UINT32_MAX, past an int (C17): a macro.
- a store's publish copied an entry rebuild left unset when the record
  was not found (the copy was never used): copied when it was made.
- the log's image writer kept a count only Linux's sync_file_range uses.
- ",{" + (row == from) read as adding an int to a string: said plainly.
- the tests: mkdtemp, hidden by _POSIX_C_SOURCE on macOS, needs
  _DARWIN_C_SOURCE; a string split so that \x80 and a stay apart is
  bracketed; a count left over and a malloc measure only glibc has.
The core's and the store's tests pass on the Mac.

1 files changed, 4 insertions, 1 deletions

diff --git a/store/store-log.c b/store/store-log.c
index f73d2bc..7b6f00c 100644
--- a/store/store-log.c
+++ b/store/store-log.c
@@ -493,7 +493,10 @@ PureResult pure_image_write(const char *directory, const char *name, bool (*next
     put64(head + 8, 0); /* its length, once known */
     Digest digest = {0};
     bool ok = write_all(fd, head, sizeof(head), 0), produced = true;
-    uint64_t at = sizeof(head), started = 0;
+    uint64_t at = sizeof(head);
+#if defined(__linux__)
+    uint64_t started = 0; /* the bytes handed to the device */
+#endif
     for (PureSpan chunk; ok && (produced = next(context, &chunk)) && chunk.size;)
         for (size_t done = 0; ok && done < chunk.size;) {
             size_t take = chunk.size - done < STEP ? chunk.size - done : STEP;