Undefined symbols for architecture x86_64: "typeinfo for std::bad_variant_access", "vtable for std::bad_variant_access"

[barracuda156]

On 10.7–10.12 absent fails to build with Clang:

Undefined symbols for architecture x86_64:
  "typeinfo for std::bad_variant_access", referenced from:
      __ZNSt3__126__throw_bad_variant_accessB6v15007Ev in and_then_test.cpp.o
      __ZNSt3__126__throw_bad_variant_accessB6v15007Ev in transform_test.cpp.o
  "vtable for std::bad_variant_access", referenced from:
      __ZNSt3__126__throw_bad_variant_accessB6v15007Ev in and_then_test.cpp.o
      __ZNSt3__126__throw_bad_variant_accessB6v15007Ev in transform_test.cpp.o
  NOTE: a missing vtable usually means the first non-inline virtual member function has no definition.
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
make[2]: *** [tests/absent_tests] Error 1

Log: https://build.macports.org/builders/ports-10.12_x86_64-builder/builds/216330/steps/install-port/logs/stdio