Fix jumbled error output when using Windows spawn fix

[RandomShaper]

In situations where the spawn fix is needed, compiler errors are reported like this because of Popen using stream mode by default:

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b"modules\\gltf\\structures\\gltf_skeleton.cpp:36:1: error: unknown type name 'intt'; did you mean 'int'?\r\nintt a;\r\n^~~~\r\nint\r\n1 error generated.\r\n"
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In contrast, with this PR, they look like this:

=====
modules\gltf\structures\gltf_skeleton.cpp:36:1: error: unknown type name 'intt'; did you mean 'int'?
intt a;
^~~~
int
1 error generated.

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[clayjohn]

Thanks!

[akien-mga]

Cherry-picked for 3.6.

[akien-mga]

Cherry-picked for 3.5.1.