Two more patches for clang.

The first adds "<prefix>/lib" to the built-in linker search path, and
the second automatically links with "-lcxxrt and "-dl" on Linux. One
now doesn't need to specify any further paths when compiling a
standard C++ program:

    > cat test.cc

    #include <iostream>

    int main(int argc, char** argv)
    {
        std::cout << "Hello, Clang!" << std::endl;
        return 0;
    }

    > clang++ --std=c++11 --stdlib=libc++ test.cc -o a.out && ./a.out
    Hello, Clang!
Adding script with a basic compiler test.