Load program binary extension on Linux

platform/default/shader_cache_tmp.cpp

@@ -0,0 +1,12 @@
+#include <mbgl/platform/platform.hpp>
+
+namespace mbgl {
+namespace platform {
+
+// Returns the path to the default shader cache on this system.
+std::string defaultShaderCache() {
+    return "/tmp/mbgl-shader-cache-";
+}
+
+}
+}