clang-tidy: avoid global non const variables

[neheb]

Found with cppcoreguidelines-avoid-non-const-global-variables

Signed-off-by: Rosen Penev [email protected]

[codecov]

Codecov Report

Merging #1674 (683950b) into main (c44c289) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #1674   +/-   ##
=======================================
  Coverage   66.94%   66.94%           
=======================================
  Files         151      151           
  Lines       20743    20743           
=======================================
  Hits        13886    13886           
  Misses       6857     6857           

Impacted Files	Coverage Δ
include/exiv2/datasets.hpp	100.00% <ø> (ø)
samples/stringto-test.cpp	90.62% <ø> (ø)
src/cr2header_int.cpp	77.14% <ø> (ø)
src/cr2header_int.hpp	50.00% <ø> (ø)
src/datasets.cpp	51.61% <ø> (ø)
src/http.cpp	0.00% <ø> (ø)
src/jpgimage.cpp	77.45% <ø> (ø)
src/properties.cpp	80.09% <ø> (ø)
src/tags.cpp	52.41% <ø> (ø)
src/tiffimage.cpp	83.47% <ø> (ø)
... and 3 more

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[neheb]

proper use of static constexpr member variables requires C++17. Bah.

[piponazo]

LGTM, Thanks!