[GHC] #3879: Enable shared libraries on Windows

[GHC]

#3879: Enable shared libraries on Windows
---------------------------------+------------------------------------------
    Reporter:  igloo             |        Owner:  benl        
        Type:  feature request   |       Status:  new         
    Priority:  high              |    Milestone:  6.14.1      
   Component:  Compiler          |      Version:  6.12.1      
    Keywords:                    |   Difficulty:              
          Os:  Unknown/Multiple  |     Testcase:              
Architecture:  Unknown/Multiple  |      Failure:  None/Unknown
---------------------------------+------------------------------------------

Comment(by benl):

 Yes, in `rts/PrimOps.cmm` there is:

 {{{
 import sm_mutex;
 }}}

 Because `sm_mutex` has an import statement it thinks this symbol is from
 some other package (dll) so is trying to use the dll's IAT (Import Address
 Table) to access it. However, `sm_mutex` is compiled into the rts package
 directly, so won't appear in the IAT, hence the undefined reference.

 This is a bit of a mess because for other reasons (I forget the exact
 details), bootstrapping via-c requires this import statement to be there.
 I actually added it myself a few weeks ago to fix the build on SPARC
 again, which was after I last built on Windows. (sigh)

 Adding a `#if defined(mingw32_HOST_OS)` around `import sm_mutex` should
 solve the problem. I've just moved house and don't have a Windows machine
 to test on right now, but I expect this will fix it.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/3879#comment:3>
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