Add hooking functionality

This allows us to hook chosen functions and do interesting things there (in
particular: reentrancy checking).

test/unit/hooks.c

@@ -0,0 +1,38 @@
+#include "test/jemalloc_test.h"
+
+static bool hook_called = false;
+
+static void
+hook() {
+	hook_called = true;
+}
+
+static int
+func_to_hook(int arg1, int arg2) {
+	return arg1 + arg2;
+}
+
+#define func_to_hook JEMALLOC_HOOK(func_to_hook, hooks_libc_hook)
+
+TEST_BEGIN(unhooked_call) {
+	hooks_libc_hook = NULL;
+	hook_called = false;
+	assert_d_eq(3, func_to_hook(1, 2), "Hooking changed return value.");
+	assert_false(hook_called, "Nulling out hook didn't take.");
+}
+TEST_END
+
+TEST_BEGIN(hooked_call) {
+	hooks_libc_hook = &hook;
+	hook_called = false;
+	assert_d_eq(3, func_to_hook(1, 2), "Hooking changed return value.");
+	assert_true(hook_called, "Hook should have executed.");
+}
+TEST_END
+
+int
+main(void) {
+	return test(
+	    unhooked_call,
+	    hooked_call);
+}