[icu] not building BUILD_DATA should still have an install target

Also, validate it's set before checking it.

Author: hyp

shared/ICU/CMakeLists.txt

@@ -546,7 +546,11 @@ target_link_libraries(icuin PRIVATE
 set_target_properties(icuin PROPERTIES
   OUTPUT_NAME icuin${PROJECT_VERSION_MAJOR})
 
-if(NOT BUILD_DATA)
+if(DEFINED BUILD_DATA AND NOT BUILD_DATA)
+  install(TARGETS icuuc icuin
+    ARCHIVE DESTINATION ${CMAKE_INSTALL_FULL_LIBDIR}
+    LIBRARY DESTINATION ${CMAKE_INSTALL_FULL_LIBDIR}
+    RUNTIME DESTINATION ${CMAKE_INSTALL_FULL_BINDIR})
   return()
 endif()