Tweaks to configure.ac

`AC_USE_SYSTEM_EXTENSIONS` takes care of defining feature_test_macros(7)
and makes sure we have a consistent environment across GHC.

This non-standard flag was used previously by GHC's build-system to set the `CC` variable.

See https://phabricator.haskell.org/D1608 for more details

Restore compat for GHC 7.8

Restoring support for GHC 7.6 would require more work due to MkFixed

This renders #41 obsolete

@AshleyYakeley I had to remove these in order to be able to update
to time-1.6 in GHC as it otherwise breaks GHC's buildsystem

GHC's buildsystem needs to regenerate those files and cause subtle but
annoying issues for users if files tracked by Git are modified everytime
GHC is built.

Fix compilation on GHC 7.0 by not using Trustworthy

use clock_gettime to implement getPOSIXTime if available

Ensure Read/Show instances are always provided with UTCTime and Universa...

Convert README file to markdown.

Loosen required cabal-version

1.14 has the same features as 1.10 so this was overly restrictive and can prevent you from installing time on older GHCs.