Supports to build maloader with clang and libc++

Since XCode4.6 compilers seems built with clang and libc++, we will see a lot of error messages that there are undefined symbols when we use maloader built with g++. To make XCode4.6 compiler, one solution is to use use maloader compiled with clang and libc++.

This patch supports to build maloader with clang and libc++.

When compiling with clang and libc++, you have to set CC and CXX, and define USE_LIBCXX=1 like the following:
CC=/path/to/clang CXX=/path/to/clang++ USE_LIBCXX=1 make

I have checked this patch works only with LLVM3.2 and Ubuntu precise.