Fix clock_gettime detection in autotools

Fix clock_gettime detection in autotools
AC_CHECK_FUNCS checks for the existence of functions in the C standard
library, so we don't need it. Instead, we need to define
HAVE_CLOCK_GETTIME if the function was found inside the librt.

Moreover, in source file check if HAVE_CLOCK_GETTIME is defined rather
than of checking if it's 0.

git-svn-id: svn+ssh://svn.enlightenment.org/var/svn/e/trunk/ecore@52863 7cbeb6ba-43b4-40fd-8cce-4c39aea84d33