Supports clang with libc++.

When maloader wants to load a binary which is built with clang and libc++, we have to compile maloader with clang and libc++. Otherwise, you will see a lot of errors that undefined symbols are found.

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