build: change reduce exports/static libstdc++ options for gitian and travis

For Gitian releases:

• Windows builds remain unchanged. libstdc++ was already linked statically.
• OSX builds remain unchanged. libstdc++ is tied to the SDK and not worth messing with.
• Linux builds now statically link libstdc++.

For Travis:

• Match the previous behavior by adding --enable-reduce-exports as necessary.
• Use static libstdc++ for the full Linux build.