[CMAKE] BOOST_TEST_DYN_LINK is defined twice
[CMAKE] BOOST_TEST_DYN_LINK is defined twice

Summary:
This is a confusing feature from `check_cxx_source_compiles`. From the
documentation related to the usage of `CMAKE_REQUIRED_DEFINITIONS`
(which is **NOT** used!):
```
A definition for the name specified by <resultVar> will also be added
automatically.
```

What this means is that the check for `BOOST_TEST_DYN_LINK` will build
with a `-DBOOST_TEST_DYN_LINK` flag, because the result variable is also
named `BOOST_TEST_DYN_LINK`. This in turns cause the check to fail with
`-Werror` due to a GCC redefinition warning being promoted to error.

This diff renames the output variable to prevent this redefinition. It
also makes the check easier to read. This has no impact on the behavior
without an additional `-Werror` flag.

Test Plan:
  cmake -GNinja .. \
    -DCMAKE_C_COMPILER=gcc -DCMAKE_CXX_COMPILER=g++ \
    -DCMAKE_C_FLAGS=-Werror -DCMAKE_CXX_FLAGS=-Werror
Look at the cmake output and check the test for `BOOST_TEST_DYN_LINK`
doesn't fail.
  ninja all check-all

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6312

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