title: How does pip know what compiler to use when building a native code extension?
body: <p>I have both Visual Studio 2017 Community and TDM-GCC both installed on my Windows 10 box.  Everything is 64-bit.  I've been trying to pip install pymqi, but so far I haven't had much luck.</p>
  
  <p>Without going into too many irrelevant details, I'm trying to get it to use TDM-GCC instead of msvc to build the C components of this package.  However, it keeps using msvc even though I don't have it set up in the command line environment I'm pip installing from (i.e. I didn't run the vcvars64.bat file).  I do, however, have TDM-GCC in the PATH.</p>
  
  <p>How does pip find the compiler?  Is there some command line option I can specify to force pip to use a specific compiler?</p>