Use feature test macro for aligned_alloc

Reorder ifdef checks in BTreeImpl::growTree to make posix_memalign the
default case and hide aligned_alloc behind feature test macro. This
covers more systems and actually fixes the compatibility with Android.

c++/src/kj/table.c++

@@ -288,27 +288,27 @@ void BTreeImpl::growTree(uint minCapacity) {
   // aligned_alloc() function. Unfortunately, many platforms don't implement it. Luckily, there
   // are usually alternatives.
 
-#if __APPLE__
-  // OSX lacks aligned_alloc(), but has posix_memalign(). Fine.
-  void* allocPtr;
-  int error = posix_memalign(&allocPtr,
-      sizeof(BTreeImpl::NodeUnion), newCapacity * sizeof(BTreeImpl::NodeUnion));
-  if (error != 0) {
-    KJ_FAIL_SYSCALL("posix_memalign", error);
-  }
-  NodeUnion* newTree = reinterpret_cast<NodeUnion*>(allocPtr);
-#elif _WIN32
+#if _WIN32
   // Windows lacks aligned_alloc() but has its own _aligned_malloc() (which requires freeing using
   // _aligned_free()).
   // WATCH OUT: The argument order for _aligned_malloc() is opposite of aligned_alloc()!
   NodeUnion* newTree = reinterpret_cast<NodeUnion*>(
       _aligned_malloc(newCapacity * sizeof(BTreeImpl::NodeUnion), sizeof(BTreeImpl::NodeUnion)));
   KJ_ASSERT(newTree != nullptr, "memory allocation failed", newCapacity);
-#else
+#elif _ISOC11_SOURCE // macro available since glibc 2.16
   // Let's use the C11 standard.
   NodeUnion* newTree = reinterpret_cast<NodeUnion*>(
       aligned_alloc(sizeof(BTreeImpl::NodeUnion), newCapacity * sizeof(BTreeImpl::NodeUnion)));
   KJ_ASSERT(newTree != nullptr, "memory allocation failed", newCapacity);
+#else
+  // OSX and Android lack aligned_alloc(), but have posix_memalign(). Fine.
+  void* allocPtr;
+  int error = posix_memalign(&allocPtr,
+      sizeof(BTreeImpl::NodeUnion), newCapacity * sizeof(BTreeImpl::NodeUnion));
+  if (error != 0) {
+    KJ_FAIL_SYSCALL("posix_memalign", error);
+  }
+  NodeUnion* newTree = reinterpret_cast<NodeUnion*>(allocPtr);
 #endif
 
   acopy(newTree, tree, treeCapacity);