Don't pass `-pthread` when building for WebAssembly.
Don't pass `-pthread` when building for WebAssembly.

Change-Id: I59c2e9598dfd48389ab72940e38372f2c324e41d
Reviewed-on: https://code-review.googlesource.com/c/re2/+/57250
Reviewed-by: Paul Wankadia <62d4e38bb1380f055ec4b899baa75e52af09c5f7@google.com>

google/re2,google/re2,google/re2,google/re2,google/re2