support clang

[t-mat]

clang don't support std::thread on Linux and Windows.
There are two solutions (same as #10)

• Wait for official library support.
• Use boost as its replacement

[t-mat]

Today, I've investigate clang 3.4-1 (svn183526-1) on Ubuntu (13.04).

• std::thread works fine.
• std::async doesn't work. (Illegal Instruction (core dumped))
  • http://llvm.org/bugs/show_bug.cgi?id=14943

[t-mat]

Add experimental branch implement-thread-pool.

• This branch only use std::thread insted of std::async.
• It seems 5%~10% slower than original.

[t-mat]

http://www.reddit.com/r/cpp_questions/comments/1o9ug3/clang_and_async/

This is an issue with the intersection of the standard library and clang.

Works: /opt/llvm/3.3/bin/clang++ -gcc-toolchain /opt/gcc/4.8.1 -std=c++11 -I /opt/boost/1.54.0/include tst.cc
Works: /opt/llvm/3.3/bin/clang++ -gcc-toolchain /opt/gcc/4.8.0 -std=c++11 -I /opt/boost/1.54.0/include tst.cc
Fails: /opt/llvm/3.3/bin/clang++ -gcc-toolchain /opt/gcc/4.7.2 -std=c++11 -I /opt/boost/1.54.0/include tst.cc
Fails: /opt/llvm/3.3/bin/clang++ -gcc-toolchain /opt/gcc/4.7.1 -std=c++11 -I /opt/boost/1.54.0/include tst.cc

[t-mat]

36d8b52 fix the compile issue for clang.
Now, we could compile with clang by the following command:

make CC=clang CXX=clang++