Incorporate OS X gettime implementation into fg_time.c

This approach has several benefits to previous implementation. First,
the number of source files are reduced. Second, the new approach doesn't
rely on __DARWIN__ (host) macros anymore. Instead, feature macros
(HAVE_CLOCK_GETTIME and HAVE_CLOCK_GET_TIME) are used, which is more
future-proof. Last, the new approach doesn't print via errx to stderr.
[Coding style] improve wording in configure.ac
Autoconf - disable asserts per default

Asserts are now disabled per default and can be enabled via ./configure
--enable-assert. As consequence, it's no longer needed to encapsulate
assert() function calls in #ifdef DEBUG macros.
Autoconf - make debug check similar to assert check