examples: update min cmake version to 3.4

[lukaszstolarczuk]

in examples using 'include' command - it also uses 'CheckIncludeFile'
which (in CMake <=3.3) worked only with set project language (CXX in this case),
and some included files are supposed to be checked with C lang, too.

With help of: https://github.com/nlohmann/cmake_min_version

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This change is 

[codecov]

Codecov Report

Merging #922 into master will decrease coverage by 0.08%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #922      +/-   ##
==========================================
- Coverage   95.99%   95.90%   -0.09%     
==========================================
  Files          48       48              
  Lines        6087     6087              
==========================================
- Hits         5843     5838       -5     
- Misses        244      249       +5     

Flag	Coverage Δ
#tests_clang_debug_cpp17	95.83% <ø> (ø)
#tests_gcc_debug	92.38% <ø> (-0.06%)	⬇️

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Impacted Files	Coverage Δ
include/libpmemobj++/mutex.hpp	79.31% <0.00%> (-6.90%)	⬇️
...ude/libpmemobj++/container/concurrent_hash_map.hpp	93.66% <0.00%> (-0.41%)	⬇️
...j++/container/detail/concurrent_skip_list_impl.hpp	98.12% <0.00%> (+0.13%)	⬆️

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[szyrom]

please update main README

Reviewed 17 of 17 files at r1.
Reviewable status: complete! all files reviewed, all discussions resolved