build: don't pass -w when building for Windows

This has been around since the introduction of autotools. However at
this point I'm not sure we'd every want to suppress all warnings when
performing a build, and given that CXX FLAGS will have been overriden
when cross-compiling for Windows (using depends), this would rarely,
if-ever be used anyways.

From https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html:
-w

    Inhibit all warning messages.

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