Fix the AKERR_USE_STDLIB=OFF build (issue #12)

The freestanding build (-DAKERR_USE_STDLIB=OFF) did not compile at all:
bool, PATH_MAX and NULL were used unconditionally in the public header
but only included under the stdlib branch, and the CMake option was
pasted straight into a preprocessor definition, so a non-numeric cache
spelling (-DAKERR_USE_STDLIB=ON) silently evaluated to 0.

- Normalize AKERR_USE_STDLIB to a plain 1/0 in CMake before stamping it,
  and use a consistent '#if AKERR_USE_STDLIB' everywhere it is tested.
- Include <stdbool.h>/<stddef.h> unconditionally (freestanding-safe);
  keep <stdlib.h>/<string.h>/<stdio.h> behind AKERR_USE_STDLIB; move
  <limits.h> out of the public header into src/error.c, its only user.
- Define the freestanding runtime contract: AKERR_RUNTIME_HEADER must
  name a header providing exit, memset, snprintf, strcmp, strlen and
  strncpy when AKERR_USE_STDLIB is OFF, or the header #errors naming
  them. Add cmake/akerr_default_runtime.h, a libc-backed convenience
  default so this repo's own OFF build and tests work out of the box.
- Route ENSURE_ERROR_READY's pool-exhaustion path through akerr_exit()
  instead of a direct exit(1). Deliberate behavior change: that exit
  code moves from 1 to AKERR_EXIT_STATUS_UNREPRESENTABLE (125), the same
  sentinel every other unrepresentable status already uses. Documented
  in docs/building.md and UPGRADING.md. Not an ABI break.
- Retire PATH_MAX: AKERR_MAX_ERROR_FNAME_LENGTH is now stamped by
  scripts/generrno.sh from a new AKERR_MAX_ERROR_FNAME_LENGTH cache
  variable, defaulting to 4096 (PATH_MAX on Linux/glibc) so
  sizeof(akerr_ErrorContext) and the soname are unchanged. Rewrite the
  now-stale PATH_MAX justification in src/lock.h's feature-test-macro
  comment.
- Fail the configure with a FATAL_ERROR, not a warning, when
  AKERR_USE_STDLIB=OFF and AKERR_THREADS would resolve to pthread,
  naming -DAKERR_THREADS=none as the fix.
- Keep the generated errno table (errno.c) out of the OFF build; skip
  the 'errno --list' shellout in scripts/generrno.sh under OFF and
  stamp AKERR_LAST_ERRNO_VALUE from a new fallback cache variable
  (default 133, Linux's EHWPOISON) instead.
- Update docs/building.md: drop the known-defect paragraph, fix
  sprintf -> snprintf, add size_t, drop PATH_MAX, and document the new
  options and the exit-code change.
- Guard tests/err_errno.c's registered-name assertion behind
  AKERR_USE_STDLIB: akerr_init_errno() is not called when it is OFF.
- Add a CI job that configures/builds/tests AKERR_USE_STDLIB=OFF with
  AKERR_THREADS=none, plus a compile-only -nostdinc -ffreestanding
  check of tests/freestanding_fixture.c against the generated header.

Bump the project version to 2.0.2 (no ABI break: soname and struct
layout are unchanged).

Verified locally: OFF+none configures and builds clean with all tests
passing; ON and the plain default build both build and pass their full
test suites (37/37); OFF with the default/auto thread backend fails
configure with a message naming -DAKERR_THREADS=none; sizeof(akerr_ErrorContext)
is unchanged (37296 bytes, fname/function still 4096 each) versus the
pre-change tree.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/error.c

@@ -1,6 +1,10 @@
 #include "akerror.h"
 #include "lock.h"
-#if defined(AKERR_USE_STDLIB) && AKERR_USE_STDLIB == 1
+/* INT_MAX (used below in akerr_reserve_status_range_locked) only, not in the
+ * public header: <limits.h> is freestanding-safe, but nothing else in this
+ * file's freestanding build needs it, so it stays out of the shared header. */
+#include <limits.h>
+#if AKERR_USE_STDLIB
 #include <stdlib.h>
 #include <stdarg.h>
 #include <stdio.h>
@@ -154,7 +158,7 @@ int akerr_valid_error_address(akerr_ErrorContext *ptr)
 
 void akerr_default_logger(const char *fmt, ...)
 {
-#if defined(AKERR_USE_STDLIB) && AKERR_USE_STDLIB == 1
+#if AKERR_USE_STDLIB
     va_list ap;
 
     va_start(ap, fmt);
@@ -285,7 +289,7 @@ static void akerr_init_state(void)
     __akerr_name_library_status(AKERR_STATUS_NAME_FOREIGN, "Foreign Status Name");
     __akerr_name_library_status(AKERR_STATUS_NAME_FULL, "Status Name Registry Full");
     __akerr_name_library_status(AKERR_STATUS_NAME_INVALID, "Invalid Status Name");
-#if (defined(AKERR_USE_STDLIB) && AKERR_USE_STDLIB == 1) || (!defined(AKERR_USE_STDLIB))
+#if AKERR_USE_STDLIB
     akerr_init_errno();
 #endif
 

src/lock.h

@@ -33,10 +33,12 @@
 
 /*
  * PTHREAD_MUTEX_RECURSIVE is XSI, so glibc hides it under a strict -std=c99
- * without _XOPEN_SOURCE. No feature-test macro is defined here, because the
- * public header already needs the same one for PATH_MAX: a build strict enough
- * to lose one has already lost the other. Build with -D_XOPEN_SOURCE=700 if you
- * need strict C99.
+ * without _XOPEN_SOURCE. No feature-test macro is defined here: this file is
+ * only ever compiled with AKERR_THREADS_PTHREAD, which CMake now refuses to
+ * pair with AKERR_USE_STDLIB=OFF (see the AKERR_THREADS/AKERR_USE_STDLIB
+ * check in CMakeLists.txt), so whatever default feature-test macros the host
+ * libc uses when building the rest of the (hosted) library apply here too.
+ * Build with -D_XOPEN_SOURCE=700 if you need strict C99.
  */
 #if defined(AKERR_THREADS_PTHREAD) && AKERR_THREADS_PTHREAD == 1