rustc generates .lib files for its libraries when compiling for Windows
(even using MinGW on Linux). But MinGW expects .a files. So we add in
rust-specific prefix and suffixes so MinGW builds can find the libs that
rustc generates. (And the RUST_LIB- variables default to the same vales
as the LIB_ variables otherwise.)

MozReview-Commit-ID: ClsA0YuJaxh

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extra : rebase_source : 7b46460c94ceb34b7a5a302ce91d3f1dca600041