Building on Windows fails depending on the result from sharding the
deps/v8/src/v8.gyp:v8_base target. If two source files with the same
name are in the same shard, their output object file path would
conflict with one another. One example of this conflict is v8_base's
runtime/runtime.cc and the V8 inspector's protocol/Runtime.cpp that
is generated at build time, for which the files runtime.obj and
Runtime.obj would be created, but msvs overwrites one of them with
the other.

Dividing the .obj output path by the original source's extension
prevents this overwrite.

Fixes: nodejs/v8#4

Refs: nodejs#13959 (comment)