added external flag for msvc

Issue/feature addressed

-isystem is not a valid flag for MSVC. /external:I is the equivalent: https://devblogs.microsoft.com/cppblog/broken-warnings-theory/

Proposed solution

if statement in CMakelists.txt to check if compiler is MSVC

Implementation

Tests

Suggested reviewers

Please suggest any people who would be appropriate to review your code.

Notes

Please add any other information that could be useful for reviewers.

Checklist

• Functions and classes, or changes to them, are documented.
• User guide/documentation is updated.
• Changelog is updated.
• Suitable tests added for new functionality.
• Contributed code is correctly formatted. (See the contributing guidelines).
• License added to any new files.
• No extraneous files have been added (e.g. compiler output or test data files).