The ckeck for ICU in regex library is detected as a build error in Visual Studio

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When building with a Makefile under Visual Studio (as a project pre-build event that compiles boost as a dependency, before the project) cl.exe compiler sees the VS_UNICODE_OUTPUT environment variable and sends the error message directly into the Visual Studio output window, even if it is just a test to check for ICU presence and the compiler outputs are otherwise redirected.

Even if boost build completes successfully, Visual Studio sees the output error message, considers the build as failed, and stops further compilation.

Unsetting VS_UNICODE_OUTPUT in the Makefile before building boost prevents the entire boost compilation process.

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Actually I find that unsetting VS_UNICODE_OUTPUT and then building will work, it was just Microsoft nmake (which I were using) that does not understand an & character on a command line, to be invoked.

So could the build process just uset VS_UNICODE_OUTPUT when testing for ICU ?