Don't check for mntent in configure script on OS X

Thanks to r1chelt (Daniel R.).

Closes #4
Add more symbols to mntent.h include line
Use new variable to set _DARWIN_C_SOURCE macro

Before $(uname) was used, which is not a really good solution.
Update ChangeLog and THANKS files about the bug
Remove 'slowfs' option for OS X

It didn't effect anything there anyway.
Use replace_string() function in four more places
Merge branch 'fix-configure-on-osx'