Only use _GLIBCXX_DEBUG on gcc/darwin.

It looks like pathscale also uses the gcc library but debug mode doesn't
work on it.

[SVN r58567]

libs/functional

@@ -1 +1 @@
-Subproject commit 7fc5d819337b0a8bc0a4752203a9d89ba42a1822
+Subproject commit 109b4d8258c59f95732f15a5910cfacaef237503