scons: Fix MSYS2 Mingw-w64 build.

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>

This patch is based on 28e3f85e09/mingw-w64-mesa/link-ole32.patch but with tweaks to avoid MSVC build break when applied.

v2: Create Mingw platform alias pointing to windows host platform define to avoid spurious crosscompilation;

v3: Fix obviously wrong compiler flags for swr driver;

v4: Update original patch URL because it has been relocated;

v5: Don't bother patching autools stuff as it's not used by MSYS2 Mingw-w64 build and it's days are numbered anyway;

v6: After Mingw posix flag fix in 295851eb things are far simpler as we don't need more linking of uuid, ole32, version and shell32 than what is already in place.

scons/llvm.py

@@ -30,6 +30,7 @@ Tool-specific initialization for LLVM
 import os
 import os.path
 import re
+import platform as host_platform
 import sys
 import distutils.version
 
@@ -192,6 +193,12 @@ def generate(env):
             'uuid',
         ])
 
+        # Mingw-w64 zlib is required when building with LLVM support in MSYS2 environment
+        if host_platform.system().lower().startswith('mingw'):
+            env.Append(LIBS = [
+                 'z',
+            ])
+
         if env['msvc']:
             # Some of the LLVM C headers use the inline keyword without
             # defining it.