Bump CXX standard to c++17. Required for ICU 75+.

--- CMakeLists.txt.orig	2019-03-26 18:44:44 UTC
+++ CMakeLists.txt
@@ -29,6 +29,7 @@ simple_option(BUILD_QCH "Build API documentation in QC
 simple_option(BUILD_QCH "Build API documentation in QCH format" OFF)
 
 # Dependencies
+set(CMAKE_CXX_STANDARD 17)
 set(REQUIRED_QT_VERSION 5.4.0)
 set(REQUIRED_KF5_VERSION 5.16.0)