Better compilation of icu4c.

- Disable icuio and layoutex.
- Hardcode UTF8 as charset (we always use utf8 and hardcoding him improve
  performances)
- Do not include default utf headers
- Do not use `using namespace icu`

See http://source.icu-project.org/repos/icu/trunk/icu4c/readme.html#RecBuild
for more information about options.

Increment base dependencies version as we are compiling icu differently.

kiwixbuild/versions.py

@@ -10,7 +10,7 @@ main_project_versions = {
 
 # This is the "version" of the whole base_deps_versions dict.
 # Change this when you change base_deps_versions.
-base_deps_meta_version = '5'
+base_deps_meta_version = '6'
 
 
 base_deps_versions = {