Add sized-delete size-checking functionality.

The existing checks are good at finding such issues (on tcache flush), but not
so good at pinpointing them.  Debug mode can find them, but sometimes debug mode
slows down a program so much that hard-to-hit bugs can take a long time to
crash.

This commit adds functionality to keep programs mostly on their fast paths,
while also checking every sized delete argument they get.

configure.ac

@@ -1492,6 +1492,23 @@ if test "x$enable_opt_safety_checks" = "x1" ; then
 fi
 AC_SUBST([enable_opt_safety_checks])
 
+dnl Look for sized-deallocation bugs while otherwise being in opt mode.
+AC_ARG_ENABLE([opt-size-checks],
+  [AS_HELP_STRING([--enable-opt-size-checks],
+  [Perform sized-deallocation argument checks, even in opt mode])],
+[if test "x$enable_opt_size_checks" = "xno" ; then
+  enable_opt_size_checks="0"
+else
+  enable_opt_size_checks="1"
+fi
+],
+[enable_opt_size_checks="0"]
+)
+if test "x$enable_opt_size_checks" = "x1" ; then
+  AC_DEFINE([JEMALLOC_OPT_SIZE_CHECKS], [ ])
+fi
+AC_SUBST([enable_opt_size_checks])
+
 JE_COMPILABLE([a program using __builtin_unreachable], [
 void foo (void) {
   __builtin_unreachable();