title: How to build multilib mingw-w64 gcc toolchain
body: <p>I used to use MSVC a lot, and now I want to try out new C++17 features, so I decided to use mingw-w64 toolchian.</p>
  
  <p>Btw I'm new to building things from source</p>
  
  <p>I also want the ability to compile both 32 bit and 64 bit application (multilib).</p>
  
  <p>I had mingw-w64 headers installed, then I tried to bootstrap GCC 7.1.0 in MSYS2.
  I configured it by this command</p>
  
  <pre><code>../src/configure --enable-languages=c,c++ --build=x86_64-w64-mingw32 --host=x86_64-w64-mingw32 --target=x86_64-w64-mingw32 --enable-targets=all --disable-werror --prefix=/c/tmp/gcc/dest --with-sysroot=/c/tmp/gcc/dest --disable-shared --disable-win32-registry --enable-version-specific-runtime-libs
  </code></pre>
  
  <p>The process completed without an error, however the compiler isn't usable.</p>
  
  <p>I tried to compile a file <code>a.cpp</code> by</p>
  
  <pre><code>g++ -o a.exe a.cpp
  </code></pre>
  
  <p>Unfortunately, a message box with title cc1plus appeared saying <code>The code execution cannot proceed because libgcc_s_seh-1.dll was not found. 
  Reinstalling the program may fix this problem.</code></p>
  
  <p>That suggested that I should have libgcc, but how can I get it?</p>