[CMAKE] Add an option to statically link libstdc++
[CMAKE] Add an option to statically link libstdc++

Summary:
The `-static-libstdc++` linker flag is passed manually through the
environment variable `LDFLAGS` when running configure `configure` for
the Linux gitian build.

Test Plan:
  cmake -DENABLE_STATIC_LIBSTDCXX=ON -GNinja ..
  ninja
  readelf -d src/bitcoind | grep libstdc++
Grep should return nothing.

  cmake -DENABLE_STATIC_LIBSTDCXX=OFF -GNinja ..
  ninja
  readelf -d src/bitcoind | grep libstdc++
Grep should return a line containing:
  Shared library: [libstdc++.so.x]

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D2966

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