Fix order of environment variables when getting system default locale

[Flamefire]

As per POSIX, see chapter 8.2, and linux man-pages, see man locale.7 the order should be

• LC_ALL
• LC_CTYPE
• LANG

This PR does this using the changes from #22

[codecov]

Codecov Report

Merging #144 (b593463) into develop (343306a) will increase coverage by 0.10%.
The diff coverage is 91.11%.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #144      +/-   ##
===========================================
+ Coverage    88.04%   88.15%   +0.10%     
===========================================
  Files          104      106       +2     
  Lines         9181     9207      +26     
===========================================
+ Hits          8083     8116      +33     
+ Misses        1098     1091       -7     

Impacted Files	Coverage Δ
include/boost/locale/format.hpp	94.32% <ø> (ø)
include/boost/locale/util.hpp	14.28% <ø> (ø)
src/boost/locale/shared/formatting.cpp	90.76% <0.00%> (ø)
test/test_ios_prop.cpp	100.00% <ø> (ø)
include/boost/locale/message.hpp	92.71% <50.00%> (ø)
src/boost/locale/util/default_locale.cpp	92.59% <91.66%> (+24.73%)	⬆️
include/boost/locale/date_time.hpp	93.98% <100.00%> (ø)
test/test_helpers.cpp	100.00% <100.00%> (ø)
test/test_util.cpp	100.00% <100.00%> (ø)

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