Revert deletion of compile flag that apparently is actually needed (see #424).

This was SVN commit r7538.

build/premake/extern_libs.lua

@@ -149,6 +149,11 @@ extern_lib_defs = {
 				tinsert(package.config["Release"].links, "libxml2")
 			else
 				pkgconfig("libxml-2.0")
+				-- libxml2 needs _REENTRANT or __MT__ for thread support;
+				-- OS X doesn't get either set by default, so do it manually
+				if OS == "macosx" then
+					tinsert(package.defines, "_REENTRANT")
+				end
 			end
 		end,
 	},