[config] Provide icu_version config value, GH #867

icu4.4 ships with an -Werror=strict-prototypes incompatible
header file. Currently we disable this warning for  src/string/encoding/shared.c
but we should rather check which icu versions ships this header, and change the
Configure logic (and order) to strip -Werror=strict-prototypes only with a bad icu_version
[config] Check icu_version for wrong uset_openEmpty() GH #867

Move step auto::warnings downwards after all probes.
See source/common/unicode/uset.h in icu.
For -Werror=strict-prototypes in src/string/encoding/shared.c.
https://ssl.icu-project.org/repos/icu/icu/tags/release-49-1/source/common/unicode/uset.h

This was broken from icu 42 until 49, fixed with 50.1
Merge branch 'smoke-me/icu_version-gh867'
[core] export Parrot_longopt_get, GH #1121

be consistent in our exported names