scons/MSYS2-MinGW-W64: Fix build options defaults

Signed-off-by: Prodea Alexandru-Liviu <liviuprodea@yahoo.com>
Reviewed-by: Jose Fonseca <jfonseca@vmware.com>
Cc: <mesa-stable@lists.freedesktop.org>

When building in a MSYS2 Mingw-w64 environment Mesa3D sets wrong default build options which inevitably lead to build failure.

(cherry picked from commit 6309c31fd8)

common.py

@@ -17,6 +17,9 @@ import SCons.Script.SConscript
 host_platform = _platform.system().lower()
 if host_platform.startswith('cygwin'):
     host_platform = 'cygwin'
+# MSYS2 default platform selection.
+if host_platform.startswith('mingw'):
+    host_platform = 'windows'
 
 # Search sys.argv[] for a "platform=foo" argument since we don't have
 # an 'env' variable at this point.
@@ -49,9 +52,18 @@ if 'PROCESSOR_ARCHITECTURE' in os.environ:
 else:
     host_machine = _platform.machine()
 host_machine = _machine_map.get(host_machine, 'generic')
+# MSYS2 default machine selection.
+if _platform.system().lower().startswith('mingw') and 'MSYSTEM' in os.environ:
+    if os.environ['MSYSTEM'] == 'MINGW32':
+        host_machine = 'x86'
+    if os.environ['MSYSTEM'] == 'MINGW64':
+        host_machine = 'x86_64'
 
 default_machine = host_machine
 default_toolchain = 'default'
+# MSYS2 default toolchain selection.
+if _platform.system().lower().startswith('mingw'):
+    default_toolchain = 'mingw'
 
 if target_platform == 'windows' and host_platform != 'windows':
     default_machine = 'x86'