Fix MinGW build

SConstruct

Index: SConstruct
diff --git a/SConstruct b/SConstruct
index 820c1a1c72aef454a6771782bebe1bf87e00c5a6..ca63c299cce947bc1d24e67b67059d12c9d9f0be 100644
--- a/SConstruct
+++ b/SConstruct
@@ -654,9 +654,18 @@ def GuessToolchain(os):
     return None
 
 
+def GuessVisibility(os, toolchain):
+  if os == 'win32' and toolchain == 'gcc':
+    # MinGW can't do it.
+    return 'default'
+  else:
+    return 'hidden'
+
+
 OS_GUESS = utils.GuessOS()
 TOOLCHAIN_GUESS = GuessToolchain(OS_GUESS)
 ARCH_GUESS = utils.GuessArchitecture()
+VISIBILITY_GUESS = GuessVisibility(OS_GUESS, TOOLCHAIN_GUESS)
 
 
 SIMPLE_OPTIONS = {
@@ -762,8 +771,8 @@ SIMPLE_OPTIONS = {
   },
   'visibility': {
     'values': ['default', 'hidden'],
-    'default': 'hidden',
-    'help': 'shared library symbol visibility'
+    'default': VISIBILITY_GUESS,
+    'help': 'shared library symbol visibility (%s)' % VISIBILITY_GUESS
   },
   'pgo': {
     'values': ['off', 'instrument', 'optimize'],