Update ICU configuration from Boost.Regex

[Flamefire]

Use the same mechanism for getting the ICU configuration as Boost.Regex does.

Error on use of ICU_LINK or ICU_LINK_LOCALE now that we have proper support.

Closes #70 as the work of @SSE4 is integrated here
Fixes #55

/cc @grafikrobot

TODO: Check that the omission of <dll-path> doesn't break anything.

[codecov[bot]]

Codecov Report

Merging #120 (cf79b51) into develop (6a9248b) will not change coverage. The diff coverage is n/a.

:exclamation: Current head cf79b51 differs from pull request most recent head bf0e7db. Consider uploading reports for the commit bf0e7db to get more accurate results

Additional details and impacted files

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[Flamefire]

@grafikrobot I'm unsure about the last commit Make icu-path non-local

Is this the correct way? Without it the variable is not set in the path_options rule and I wasn't able to find information about local/scopes in B2

[grafikrobot]

@grafikrobot I'm unsure about the last commit Make icu-path non-local

Is this the correct way? Without it the variable is not set in the path_options rule and I wasn't able to find information about local/scopes in B2

That's correct. The local var only lasts as long as the evaluated scope of the jamfile. And since it's evaluated once. And afterwards the rule is called the var is gone. One convention we do use is to prefix module global variables with a period ("."). I.e. .icu-path = .... To designate that they are actually private vars.