Don't specify `-pthread` on Emscripten either.

Change-Id: I6b23bb04f0caaa2a65a23c941b1aea1eeb4da25d
Reviewed-on: https://code-review.googlesource.com/c/re2/+/62913
Reviewed-by: Ash Liu <almquist@google.com>
Reviewed-by: Paul Wankadia <junyer@google.com>
1 file changed