The part that put me on my (as I now think) wrong track is secton 3.2 of the EULA. A Redistributable is the sample code, a library or a header file (defined in 3.1).

3.2 If you use the Redistributables, then in addition to your compliance with the applicable distribution requirements described for the Redistributables, the following also applies. Your license rights to the Redistributables are conditioned upon your not (a) creating derivative works of the Redistributables in any manner that would cause the Redistributables in whole or in part to become subject to any of the terms of an Excluded License; and (b) distributing the Redistributables (or derivative works thereof) in any manner that would cause the Redistributables to become subject to any of the terms of an Excluded License. An “Excluded License” is any license which requires as a condition of use, modification and/or distribution of software subject to the Excluded License, that such software or other software combined and/or distributed with such software (x) be disclosed or distributed in source code form; (y) be licensed for the purpose of making derivative works; or (z) be redistributable at no charge.

So, if I remember correctly, the GPL especially does not put the C runtime under its infection and thus it's possible to compile binaries that fall under an Excluded License even if they use the C runtime, as long as the C runtime itself is not forced under the Excluded License.

Of course, for "home use", that is, compiling Perl and compiling XS for Perl, which is what I see the tools mostly for, this point is moot, as nothing will be redistributed anyway.

You remember mostly correctly.

The GPL says nothing about whether a C runtime has been GPLed. The relevant section is part of section 0: Activities other than copying, distribution and modification are not covered by this License; they are outside its scope. The act of running the Program is not restricted, and the output from the Program is covered only if its contents constitute a work based on the Program (independent of having been made by running the Program). Whether that is true depends on what the Program does. However gcc is designed so that the binary it outputs does not incorporate copyrighted GPLed material. Therefore you can use gcc to compile non-GPLed material.

Whether you can legally use Microsoft's compiler to compile GPLed software depends on whether the output includes material that Microsoft has copyright on. The answer to that is not easily discerned.

that would cause the Redistributables in whole or in part to become subject to any of the terms of an Excluded License

But the 'redistributables' will not be affected thus. There is no licence in the world that causes Microsoft's intellectual property to be subject to it without Microsoft's permission. The word they are looking for is 'require', not 'cause'. This does not exclude the GPL because the GPL does not supersede copyright law.

Update: What would actually occur is a violation of the 'excluded licence'.