remove protobuf_c_default_allocator

"protobuf_c_default_allocator" is the last bit of global data in
libprotobuf-c, and it can be hidden with some relatively easy (though
API/ABI-breaking) changes to ProtobufCBufferSimple.

all exported functions that take a ProtobufCAllocator either use the
provided allocator (if non-NULL), or switch to the default allocator (if
NULL). there are now two relatively unambiguous choices when a
ProtobufCAllocator is required by an exported function:

    1) use NULL. the vast majority of callers should pick this option.

    2) implement your own allocation functions, and enclose these in a
    ProtobufCAllocator struct.

in previous versions of protobuf-c there were two other possibilities:

    3) maybe use &protobuf_c_system_allocator?

    4) maybe use &protobuf_c_default_allocator?

this was relatively confusing, and by removing these latter two options
we can avoid having global library state.

protobuf-c/libprotobuf-c.sym

 LIBPROTOBUF_C_1.0.0 {
 global:
         protobuf_c_buffer_simple_append;
-        protobuf_c_default_allocator;
         protobuf_c_enum_descriptor_get_value;
         protobuf_c_enum_descriptor_get_value_by_name;
         protobuf_c_message_check;

protobuf-c/protobuf-c.c

@@ -164,11 +164,11 @@ do_free(ProtobufCAllocator *allocator, void *data)
 }
 
 /*
- * Some users may configure the default allocator; providing your own allocator
- * to unpack() is preferred. This allocator is still used for packing nested
- * messages.
+ * This allocator uses the system's malloc() and free(). It is the default
+ * allocator used if NULL is passed as the ProtobufCAllocator to an exported
+ * function.
  */
-ProtobufCAllocator protobuf_c_default_allocator = {
+static ProtobufCAllocator protobuf_c__allocator = {
 	.alloc = &system_alloc,
 	.free = &system_free,
 	.allocator_data = NULL,
@@ -184,16 +184,20 @@ protobuf_c_buffer_simple_append(ProtobufCBuffer *buffer,
 	size_t new_len = simp->len + len;
 
 	if (new_len > simp->alloced) {
+		ProtobufCAllocator *allocator = simp->allocator;
 		size_t new_alloced = simp->alloced * 2;
 		uint8_t *new_data;
+
+		if (allocator == NULL)
+			allocator = &protobuf_c__allocator;
 		while (new_alloced < new_len)
 			new_alloced += new_alloced;
-		new_data = do_alloc(&protobuf_c_default_allocator, new_alloced);
+		new_data = do_alloc(allocator, new_alloced);
 		if (!new_data)
 			return;
 		memcpy(new_data, simp->data, simp->len);
 		if (simp->must_free_data)
-			do_free(&protobuf_c_default_allocator, simp->data);
+			do_free(allocator, simp->data);
 		else
 			simp->must_free_data = TRUE;
 		simp->data = new_data;
@@ -2692,7 +2696,7 @@ protobuf_c_message_unpack(const ProtobufCMessageDescriptor *desc,
 	ASSERT_IS_MESSAGE_DESCRIPTOR(desc);
 
 	if (allocator == NULL)
-		allocator = &protobuf_c_default_allocator;
+		allocator = &protobuf_c__allocator;
 
 	rv = do_alloc(allocator, desc->sizeof_message);
 	if (!rv)
@@ -2960,7 +2964,7 @@ protobuf_c_message_free_unpacked(ProtobufCMessage *message,
 
 	ASSERT_IS_MESSAGE(message);
 	if (allocator == NULL)
-		allocator = &protobuf_c_default_allocator;
+		allocator = &protobuf_c__allocator;
 	message->descriptor = NULL;
 	for (f = 0; f < desc->n_fields; f++) {
 		if (desc->fields[f].label == PROTOBUF_C_LABEL_REPEATED) {

protobuf-c/protobuf-c.h

@@ -458,6 +458,8 @@ struct ProtobufCBufferSimple {
 	uint8_t			*data;
 	/** Whether `data` must be freed. */
 	protobuf_c_boolean	must_free_data;
+	/** Allocator to use. May be NULL to indicate the system allocator. */
+	ProtobufCAllocator	*allocator;
 };
 
 /**
@@ -736,17 +738,6 @@ struct ProtobufCServiceDescriptor {
 	const unsigned			*method_indices_by_name;
 };
 
-/**
- * The default memory allocator.
- *
- * By default, it uses the system allocator (meaning malloc() and free()).
- * This is typically changed to adapt to frameworks that provide
- * some non-standard allocation functions.
- *
- * NOTE: you may modify this allocator.
- */
-extern PROTOBUF_C__API ProtobufCAllocator protobuf_c_default_allocator;
-
 /**
  * Get the version of the protobuf-c library. Note that this is the version of
  * the library linked against, not the version of the headers compiled against.
@@ -1028,7 +1019,8 @@ protobuf_c_service_descriptor_get_method_by_name(
 	sizeof(array_of_bytes),                                         \
 	0,                                                              \
 	(array_of_bytes),                                               \
-	0                                                               \
+	0,                                                              \
+	NULL                                                            \
 }
 
 /**
@@ -1037,9 +1029,12 @@ protobuf_c_service_descriptor_get_method_by_name(
 #define PROTOBUF_C_BUFFER_SIMPLE_CLEAR(simp_buf)                        \
 do {                                                                    \
 	if ((simp_buf)->must_free_data) {                               \
-		protobuf_c_default_allocator.free(                      \
-			&protobuf_c_default_allocator,                  \
+		if ((simp_buf)->allocator != NULL)                      \
+			(simp_buf)->allocator->free(                    \
+				(simp_buf)->allocator,                  \
 				(simp_buf)->data);			\
+		else                                                    \
+			free((simp_buf)->data);                         \
 	}                                                               \
 } while (0)