Default --march-tune-native to false

Despite the warnings, people still build using a different environment than the deployment one.
This doesn't actually make the built library portable, just less likely to crash on a different CPU.

Refs #146

Author: glebm

ext/extconf.rb

@@ -19,8 +19,7 @@
   $LDFLAGS << ' -static-libgcc -static-libstdc++'
 end
 
-# Set to false when building binary gems
-if enable_config('march-tune-native', true)
+if enable_config('march-tune-native', false)
   $CFLAGS << ' -march=native -mtune=native'
   $CXXFLAGS << ' -march=native -mtune=native'
 end