Build fails when multiple architectures of libc are installed

I'm trying to add triton to my ctf-tools repo (https://github.com/zardus/ctf-tools), and am getting the following error:

``` ShellSession
yans@lath ~/tools/triton/pin-2.14-71313-gcc.4.4.7-linux/source/tools/Triton/build $ cmake ..
CMake Error at CMakeLists.txt:59 (if):
  if given arguments:

    "NOT" "EXISTS" "/usr/include/x86_64-linux-gnu/asm/unistd_64.h" "/usr/include/i386-linux-gnu/asm/unistd_64.h"

  Unknown arguments specified


-- Configuring incomplete, errors occurred!
See also "/home/yans/tools/triton/pin-2.14-71313-gcc.4.4.7-linux/source/tools/Triton/build/CMakeFiles/CMakeOutput.log".
```

This seems to be because I have both `linux-libc-dev:amd64` and `linux-libc-dev:i386` installed (under Ubuntu 14.04), and they each contain a `unistd_64.h` file. In turn, your recursive check for unistd_64.h returns both files, which confuses cmake.

I'll open a PR with a hackish fix, in case that's good enough.

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