Use proper cmake settings for C++17 mode instead of gradle cppflags

libsession-util/build.gradle

@@ -14,7 +14,6 @@ android {
         testInstrumentationRunner "androidx.test.runner.AndroidJUnitRunner"
         externalNativeBuild {
             cmake {
-                cppFlags "--std=c++17"
             }
         }
 //        ndk {

libsession-util/src/main/cpp/CMakeLists.txt

@@ -9,6 +9,11 @@ cmake_minimum_required(VERSION 3.18.1)
 
 project("session_util")
 
+# Compiles in C++17 mode
+set(CMAKE_CXX_STANDARD 17)
+set(CMAKE_CXX_STANDARD_REQUIRED ON)
+set(CMAKE_CXX_EXTENSIONS OFF)
+
 # Creates and names a library, sets it as either STATIC
 # or SHARED, and provides the relative paths to its source code.
 # You can define multiple libraries, and CMake builds them for you.