fix MSVC -isystem flag incompatibility

MSVC doesn't understand the GCC/Clang -isystem flag. It interprets
it as the -i option (force include) + "system" + path as source file.

Use /external:I for MSVC instead, which is the MSVC equivalent for
marking a directory as an external/system include path.

Co-Authored-By: Claude Opus 4.5 <[email protected]>

Author: rok