Fix compilation of spidermonkey-17 with clang

Gentoo-Bug: https://bugs.gentoo.org/show_bug.cgi?id=458142
Upstream-Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=887645

b/js/src/js-config.in
--- a/js/src/jsinterp.cpp	2016-06-03 00:08:04.376312080 -0500
+++ b/js/src/jsinterp.cpp	2016-06-03 00:12:17.740329180 -0500
@@ -3664,7 +3664,9 @@
 BEGIN_CASE(JSOP_LEAVEFORLETIN)
 BEGIN_CASE(JSOP_LEAVEBLOCKEXPR)
 {
+    #ifdef DEBUG
     DebugOnly<uint32_t> blockDepth = regs.fp()->blockChain().stackDepth();
+    #endif
 
     regs.fp()->popBlock(cx);