feat: add CMake option to disable -Werror/-WX

[ghost]

Fixes #671

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This change is 

[codecov]

Codecov Report

Merging #1150 into master will increase coverage by 0.05%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1150      +/-   ##
==========================================
+ Coverage    93.6%   93.66%   +0.05%     
==========================================
  Files         169      169              
  Lines       13106    13109       +3     
==========================================
+ Hits        12268    12278      +10     
+ Misses        838      831       -7

Impacted Files	Coverage Δ
google/cloud/spanner/internal/polling_loop.h	91.89% <0%> (-2.71%)	⬇️
.../spanner/benchmarks/multiple_rows_cpu_benchmark.cc	64.05% <0%> (ø)	⬆️
...anner/integration_tests/client_integration_test.cc	98.97% <0%> (ø)	⬆️
...on_tests/rpc_failure_threshold_integration_test.cc	85.71% <0%> (+0.15%)	⬆️
google/cloud/spanner/samples/samples.cc	86.37% <0%> (+0.28%)	⬆️
google/cloud/spanner/client.cc	90.51% <0%> (+0.86%)	⬆️
google/cloud/spanner/internal/spanner_stub.cc	69.23% <0%> (+1.09%)	⬆️
...ud/spanner/integration_tests/client_stress_test.cc	83.33% <0%> (+1.75%)	⬆️
...loud/spanner/internal/partial_result_set_resume.cc	95.45% <0%> (+4.54%)	⬆️

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