[1.5] Do not check pkgconfig on windows

[igchor]

it might be installed (as part of cygwin).

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This change is 

[pmem-bot]

@ldorau and @annamarcink please review this pull request

[ldorau]

Reviewed 6 of 6 files at r1.
Reviewable status: complete! all files reviewed, all discussions resolved

[lukaszstolarczuk]

tbh, I don't get it - "it might be installed" so don't check it...?

Reviewable status: complete! all files reviewed, all discussions resolved

[igchor]

It might be installed on windows but it's not what we want. It could be a part of cygwin for example, and we cannot use that. The only option is to never use pkg config on windows.

Reviewable status: complete! all files reviewed, all discussions resolved

[lukaszstolarczuk]

ok, thanks, so perhaps update the commit message, please

[ldorau]

Please rebase, because Appveyor build is failing

Reviewable status: complete! all files reviewed, all discussions resolved

[codecov]

Codecov Report

Merging #381 into stable-1.5 will decrease coverage by 2.06%.
The diff coverage is n/a.

@@              Coverage Diff               @@
##           stable-1.5     #381      +/-   ##
==============================================
- Coverage       90.76%   88.69%   -2.07%     
==============================================
  Files              26       26              
  Lines            1050      867     -183     
==============================================
- Hits              953      769     -184     
- Misses             97       98       +1

Flag	Coverage Δ
#tests_clang_debug_cpp17	91.44% <ø> (ø)	⬆️
#tests_clang_release	90.31% <ø> (ø)	⬆️
#tests_gcc_debug	?
#tests_gcc_release_cpp17	89.39% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
include/libpmemobj++/condition_variable.hpp	81.15% <0%> (-4.18%)	⬇️
include/libpmemobj++/mutex.hpp	62.96% <0%> (-2.56%)	⬇️
include/libpmemobj++/shared_mutex.hpp	61.9% <0%> (-2.54%)	⬇️
include/libpmemobj++/pool.hpp	89.58% <0%> (-2.49%)	⬇️
include/libpmemobj++/p.hpp	92.3% <0%> (-2.14%)	⬇️
include/libpmemobj++/allocator.hpp	84.61% <0%> (-2.06%)	⬇️
include/libpmemobj++/persistent_ptr.hpp	93.67% <0%> (-1.79%)	⬇️
include/libpmemobj++/detail/common.hpp	81.81% <0%> (-1.52%)	⬇️
include/libpmemobj++/experimental/v.hpp	94.44% <0%> (-1.02%)	⬇️
include/libpmemobj++/timed_mutex.hpp	71.42% <0%> (-0.67%)	⬇️
... and 8 more

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