Add minimal mappings for libc++

This mapping file can be used to work around issues #141, #299,

It doesn't really fix the issues, but it allows users to get on
with their lives.

Patch adapted from work by Eugene Zelenko, thanks!

libcxx.imp

@@ -0,0 +1,7 @@
+# libc++ headers
+[
+  { include: ["<__functional_base>", private, "<functional>", public ] },
+  { include: ["<__mutex_base>", private, "<mutex>", public ] },
+  { symbol: [ "std::move", private, "<algorithm>", public ] },
+  { symbol: [ "std::string", private, "<string>", public ] },
+]