add android support (#171)

* adjust for android

* update docs

* add const qualifier to `alloc_size`

* check const qualifier in cmake

src/mem/alloc.h

@@ -457,7 +457,7 @@ namespace snmalloc
       return pointer_cast<void>(external_address<location>(p));
     }
 
-    static size_t alloc_size(void* p)
+    static size_t alloc_size(const void* p)
     {
       // This must be called on an external pointer.
       size_t size = ChunkMap::get(address_cast(p));

src/mem/mediumslab.h

@@ -39,9 +39,10 @@ namespace snmalloc
       return OS_PAGE_SIZE;
     }
 
-    static Mediumslab* get(void* p)
+    static Mediumslab* get(const void* p)
     {
-      return pointer_align_down<SUPERSLAB_SIZE, Mediumslab>(p);
+      return pointer_align_down<SUPERSLAB_SIZE, Mediumslab>(
+        const_cast<void*>(p));
     }
 
     void init(RemoteAllocator* alloc, sizeclass_t sc, size_t rsize)

src/mem/metaslab.h

@@ -140,9 +140,9 @@ namespace snmalloc
       return (slab_end - allocation_start) % size == 0;
     }
 
-    static Slab* get_slab(void* p)
+    static Slab* get_slab(const void* p)
     {
-      return pointer_align_down<SLAB_SIZE, Slab>(p);
+      return pointer_align_down<SLAB_SIZE, Slab>(const_cast<void*>(p));
     }
 
     static bool is_short(Slab* p)

src/mem/superslab.h

@@ -65,9 +65,10 @@ namespace snmalloc
       StatusChange = 2
     };
 
-    static Superslab* get(void* p)
+    static Superslab* get(const void* p)
     {
-      return pointer_align_down<SUPERSLAB_SIZE, Superslab>(p);
+      return pointer_align_down<SUPERSLAB_SIZE, Superslab>(
+        const_cast<void*>(p));
     }
 
     static bool is_short_sizeclass(sizeclass_t sizeclass)

src/override/malloc.cc

@@ -14,6 +14,10 @@ using namespace snmalloc;
 #  define SNMALLOC_NAME_MANGLE(a) a
 #endif
 
+#ifndef MALLOC_USABLE_SIZE_QUALIFIER
+#  define MALLOC_USABLE_SIZE_QUALIFIER
+#endif
+
 extern "C"
 {
   SNMALLOC_EXPORT void* SNMALLOC_NAME_MANGLE(__malloc_end_pointer)(void* ptr)
@@ -43,7 +47,9 @@ extern "C"
     return ThreadAlloc::get_noncachable()->alloc<ZeroMem::YesZero>(sz);
   }
 
-  SNMALLOC_EXPORT size_t SNMALLOC_NAME_MANGLE(malloc_usable_size)(void* ptr)
+  SNMALLOC_EXPORT
+  size_t SNMALLOC_NAME_MANGLE(malloc_usable_size)(
+    MALLOC_USABLE_SIZE_QUALIFIER void* ptr)
   {
     return Alloc::alloc_size(ptr);
   }

src/pal/pal_posix.h

@@ -2,8 +2,10 @@
 
 #include "../ds/address.h"
 #include "../mem/allocconfig.h"
-
-#include <execinfo.h>
+#if __has_include(<execinfo.h>)
+#  define SNMALLOC_HAS_BACKTRACE 1
+#  include <execinfo.h>
+#endif
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
@@ -40,6 +42,7 @@ namespace snmalloc
 
     static void print_stack_trace()
     {
+#ifdef SNMALLOC_HAS_BACKTRACE
       constexpr int SIZE = 1024;
       void* buffer[SIZE];
       auto nptrs = backtrace(buffer, SIZE);
@@ -47,6 +50,7 @@ namespace snmalloc
       backtrace_symbols_fd(buffer, nptrs, STDOUT_FILENO);
       puts("");
       fflush(stdout);
+#endif
     }
 
     /**