[v3] Fix / clean-up config vs crosses (4/n) + librt improvements

[Paolo Carlini]

Hi,

the below first moves  GLIBCXX_CHECK_GETTIMEOFDAY to the set of checks 
in common between native and crosses, by changing it to use 
GCC_TRY_COMPILE_OR_LINK.

Less trivially, it also changes GLIBCXX_CHECK_CLOCK_GETTIME to a 
flexible configure-time option, [no] be default because in general it 
requires TRY_LINK tests, but with the possibility of searching libposix4 
and also librt. The latter seems to me a good improvement, because the 
default on many common platforms (e.g., linux) is the same (no 
clock_gettime facilities), but in case of need, if the user knows what 
is doing, all the available clocks can be easily configured and made 
available to the <chrono> implementation.

Tested x86_64-linux various ways... Committed to mainline.

Paolo.

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