Disable GET_TIME on Mac.

- This should fix the Mac version not compiling in issue #85.
Remove usages of GCC __attribute__.

- To allow clang to compile, as it doesn't according to #58.
- The target attributes are not needed anymore due to makefile handling
  targetting now.
- The __attribute__((unused)) used for debugging. I don't know if clang
  supports this attribute or not but it doesn't seem very important so
  I'm removing it just in case.
Fix incorrect const qualifiers.

- Change input pointers to const in dct-generic, like they should have been.
- Fixes compilation error on GCC.