Simplify #ifdef statements.
readpin: Drop tunetime option and simplify option handling.

Tunetime was not portable to e.g. MacOSX and only gained 1% on my Raspberry Pi when compiled non-optimized with clang 3.3
It was also not part of the library.
README: adjust for tunetime removal in readpin.
readpin: Re-indent and merge two printf statements.
Test for and mention compatibility with MacOS and Cygwin, and mention state of GPIO support.