Add timespec_get(3) in <time.h> and enable unconditionally struct timespec

These changes conforms to the C11 standard
References:
- 7.27.1/3 Components of time (struct timespec)
- 7.27.2.5 The timespec_get function

According to ISO/IEC 9899:201x (draft) <time.h> defines the timespec
structure and declares the timespec_get(3) function with TIME_UTC
definition.

According to a C++17 standard draft <ctime> offers the same interface in
the std:: namespace.

The timespec_get function modifies the timespec object pointed by ts
to hold the current calendar time in the given base. The standard notes
only the TIME_UTC base with implementation defined value, set it to 1
as zero is reserved for error handling. Once operation was successful this
function returns passed base, otherwise exits with zero.

The timespec struct was already part of the POSIX standard in <time.h>.

Enable this interface unconditionally in the header to allow to use it
in a code prior C11 and C++17 as an extension.

Review notes from <christos>

(kamil)