Set SONAME to 6.
Set SONAME to 6.

Suggested by Stefano Rivera and endorsed by Evangelos Foutras in
dicussion on the GitHub issue, 6 is considered to be larger than
what any distribution is currently using.

Fixes #243.

Change-Id: I92185ccd3109123a3ed3e55693f475a74bc2a9b2
Reviewed-on: https://code-review.googlesource.com/c/re2/+/52738
Reviewed-by: Paul Wankadia <62d4e38bb1380f055ec4b899baa75e52af09c5f7@google.com>

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