llvm: Disable pthread on mingw

llvm supports both win32 native threads and pthread,
but configure tries to find pthread first.
This manually disables pthread to use native api.

This removes libpthreads-2.dll dependency on librustc.

configure

@@ -910,9 +910,11 @@ do
         # Try to have LLVM pull in as few dependencies as possible (#9397)
         LLVM_OPTS="$LLVM_OPTS --disable-zlib --disable-libffi"
 
-        # pthreads works badly on mingw-w64 systems: #8996
+        # Use win32 native thread/lock apis instead of pthread wrapper.
+        # (llvm's configure tries to find pthread first, so we have to disable it explicitly.)
+        # Also note that pthreads works badly on mingw-w64 systems: #8996
         case "$CFG_BUILD" in
-            (*w64-mingw32)
+            (*-mingw32)
             LLVM_OPTS="$LLVM_OPTS --disable-pthreads"
             ;;
         esac

src/etc/mklldeps.py

@@ -53,6 +53,5 @@
         lib = lib[2:] # chop of the leading '-l'
         print "#[link(name = \"" + lib + "\", kind = \"static\")]"
     if os == 'win32':
-        print "#[link(name = \"pthread\")]"
         print "#[link(name = \"imagehlp\")]"
     print "extern {}"

src/etc/snapshot.py

@@ -32,8 +32,7 @@ def scrub(b):
     }
 
 winnt_runtime_deps = ["libgcc_s_dw2-1.dll",
-                      "libstdc++-6.dll",
-                      "libpthread-2.dll"]
+                      "libstdc++-6.dll"]
 
 def parse_line(n, line):
   global snapshotfile