Fix:

error: use of undeclared identifier 'MAP_ANONYMOUS'

https://github.com/r-spatial/s2/issues/245
--- a/src/absl/debugging/internal/examine_stack.cc	2023-05-13 15:29:12.000000000 -0500
+++ b/src/absl/debugging/internal/examine_stack.cc	2023-07-29 04:13:04.000000000 -0500
@@ -54,6 +54,9 @@
 // Async-signal safe mmap allocator.
 void* Allocate(size_t num_bytes) {
 #ifdef ABSL_HAVE_MMAP
+#if defined(MAP_ANON) && !defined(MAP_ANONYMOUS)
+#define MAP_ANONYMOUS MAP_ANON
+#endif
   void* p = ::mmap(nullptr, num_bytes, PROT_READ | PROT_WRITE,
                    MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
   return p == MAP_FAILED ? nullptr : p;
--- a/src/absl/debugging/internal/stack_consumption.cc	2023-05-13 15:29:12.000000000 -0500
+++ b/src/absl/debugging/internal/stack_consumption.cc	2023-07-29 04:12:12.000000000 -0500
@@ -110,6 +110,9 @@
   // current stack pointer to figure out what thread is currently running.
   // Therefore, the alternate stack must be allocated from the main stack
   // itself.
+#if defined(MAP_ANON) && !defined(MAP_ANONYMOUS)
+#define MAP_ANONYMOUS MAP_ANON
+#endif
   void* altstack = mmap(nullptr, kAlternateStackSize, PROT_READ | PROT_WRITE,
                         MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
   ABSL_RAW_CHECK(altstack != MAP_FAILED, "mmap() failed");