[1.6] Add option to force define IS_TRIVIALLY_COPYABLE

[igchor]

This change is 

[pmem-bot]

@kkajrewicz and @wlemkows please review this pull request

[szyrom]

btw. we should add CI runs with gcc 4.85 AND with clang 3.3

Reviewed 2 of 2 files at r1.
Reviewable status: complete! all files reviewed, all discussions resolved

[codecov]

Codecov Report

Merging #325 into stable-1.6 will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           stable-1.6    #325   +/-   ##
==========================================
  Coverage        96.5%   96.5%           
==========================================
  Files              30      30           
  Lines            2091    2091           
==========================================
  Hits             2018    2018           
  Misses             73      73

Flag	Coverage Δ
#tests_clang_debug_cpp17	96.93% <ø> (ø)	⬆️
#tests_gcc_debug	97.88% <ø> (ø)	⬆️
#tests_gcc_release_cpp17_no_valgrind	96.63% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
include/libpmemobj++/detail/common.hpp	93.54% <ø> (ø)	⬆️
include/libpmemobj++/transaction.hpp	93.97% <ø> (ø)	⬆️

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[kkajrewicz]

Reviewed 2 of 2 files at r1.
Reviewable status: complete! all files reviewed, all discussions resolved