Require regex-posix-clib on windows directly (fixes #4, #7)

[andreasabel]

Require regex-posix-clib on windows directly (fix #4, fix #7).

Also:

• switch package to stable
• haddock cosmetics
• source code cosmetics
• remove -O2 in cabal file
• add -Wcompat in cabal file

Release candidate at: https://hackage.haskell.org/package/regex-posix-0.96.0.1/candidate/

TODO:

• wait for regex-posix-clib on stackage
• add release date to CHANGELOG
• release to hackage
• tag 0.96.0.1
• wait for stackage nightly to pick it up
• test stack-build from nightly

[andreasabel]

Testing stack-building this PR under Windows was positive.

With regex-posix-clib on stackage (commercialhaskell/stackage#6118) this should stack-build out-of-the-box.