Stop delivering a linkable library as this is header only since some time

[Romain-Geissler-1A]

Hi,

Similarly to boostorg/system#132, since the doc mentions that Boost.Regex is header only except for the ones needing the deprecated POSIX API, would it make sense nowadays to drop the deprecated POSIX API entirely and stop building any linkable library libboost_regex.so/a ?

Thanks,
Romain

[Romain-Geissler-1A]

Implemented via #256