Question about the 1.76.0 regex with ICU support

[wangqr]

Upgrading boost from 1.75.0 -> 1.76.0, regex becomes header only. Now if we use boost/regex/icu.hpp in 1.76.0, we'll need to link to icuin, otherwise we get a undefined reference to symbol error for _ZN6icu_698Collator14createInstanceERKNS_6LocaleER10UErrorCode.

Can we either:

• Add something like target_link_libraries(boost_regex INTERFACE ICU::in) to the installed boost_regex-config.cmake if BOOST_REGEX_USE_ICU is defined when configure, or
• Update the document (eg. https://www.boost.org/doc/libs/1_76_0/libs/regex/doc/html/boost_regex/install.html#boost_regex.install.building_with_unicode_and_icu_su), add instructions to link to icu when using boost::regex with icu support.

Ref: wangqr/Aegisub#113

[jzmaddock]

Addressed in #161