MinGW Makefiles: Quote UNC paths on command lines

Ed Hartleyrequested to merge
ehartley/c-make-fork:mingw-unc into master

UNC paths (starting with \\) need quotes when generating MinGW Makefiles to avoid gmake interpreting the first \ as an escape character.

Fixes: #24061 (closed)
Backport: release

Edited by Brad King

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