Prepare for ICU update

- libicule and libiculx do not exist anymore in newer ICU versions
  (harfbuzz replaces them), but we didn't actually use them, so remove
  them from the build feature and from the package dependencies
- Add namespace usage marcos since the newer ICU packages put ICU things
  in a namespace, making it easier to have multiple versions of ICU used
  side by side.

No functional change intended, but this makes it possible to build the
code with either ICU 57 (for gcc2) or 66 (for other architectures).

build/jam/BuildFeatures

@@ -104,8 +104,22 @@ if [ IsPackageAvailable icu_devel ] {
 				$(developLibDir)/libicudata.so
 				$(developLibDir)/libicui18n.so
 				$(developLibDir)/libicuio.so
-				$(developLibDir)/libicule.so
-				$(developLibDir)/libiculx.so
+				$(developLibDir)/libicutu.so
+				$(developLibDir)/libicuuc.so
+			headers: $(developHeadersDir)
+		;
+
+	EnableBuildFeatures icu ;
+} else if [ IsPackageAvailable icu66_devel ] {
+	ExtractBuildFeatureArchives icu :
+		file: base icu66
+			runtime: lib
+		file: devel icu66_devel
+			depends: base
+			libraries:
+				$(developLibDir)/libicudata.so
+				$(developLibDir)/libicui18n.so
+				$(developLibDir)/libicuio.so
 				$(developLibDir)/libicutu.so
 				$(developLibDir)/libicuuc.so
 			headers: $(developHeadersDir)