C++ Cmake doesn't set compiler language standard on Travis CI server - c++

Question

I am developing an own open source programming language interpreter project and I can make silly mistakes as I am not sophisticated in Cmake and I am trying Travis CI for the first time. My project compiles on my PC (running Ubuntu 16.04LTS, GCC 8.1 and Cmake 3.12.0-rc2) from bash terminal by executing scripts/build_all.sh and tests are executed by scripts/run_tests.sh.
Build script creates a cmake-build-debug directory, runs cmake and than builds and executes simple cmake-build-debug/lexertl_gen/lexert_gen.out program to generate a header file in my include folder for the main project. After that it builds the main project alongside with it's tests all under compiler option std=gnu++2a.
Here are all my CMakeLists.txt (the problem is described bellow):
Root CMakeLists.txt
cmake_minimum_required(VERSION 3.9.0)
set(INCLUDE_ROOT "/usr/local/include")
set(BOOST_ROOT "${INCLUDE_ROOT}/boost_1_67_0")
set(LEXERTL_ROOT "${INCLUDE_ROOT}/lexertl")
set(GTEST_ROOT "${INCLUDE_ROOT}/googletest")
set(CMAKE_CXX_FLAGS "-g -std=gnu++2a -Werror \
-Wno-error=variadic-macros \
-Werror=implicit-fallthrough")
file(GLOB SOURCES "src/*.cpp")
include_directories(include ${BOOST_ROOT} ${LEXERTL_ROOT} ${GTEST_ROOT})
add_library(calamity_core ${SOURCES})
project(lexer.out)
add_executable(${PROJECT_NAME} main_lexer.cpp)
target_link_libraries(${PROJECT_NAME} m calamity_core)
project(parser.out)
add_executable(${PROJECT_NAME} main_parser.cpp)
target_link_libraries(${PROJECT_NAME} m calamity_core)
project(tests.out)
add_subdirectory(tests)
project(lexertl_gen.out)
add_subdirectory(lexertl_gen)
tests/CMakeLists.txt
cmake_minimum_required(VERSION 3.9.0)
set(CMAKE_CXX_FLAGS "-g -std=gnu++2a -Werror \
-Wno-error=variadic-macros \
-Werror=implicit-fallthrough")
add_subdirectory(/usr/local/include/googletest ../gtets-build)
add_executable(${PROJECT_NAME} tests.cpp)
target_link_libraries(${PROJECT_NAME} gtest gtest_main gmock calamity_core)
lexertl_gen/CMakeLists.txt
cmake_minimum_required(VERSION 3.9.0)
set(CMAKE_CXX_FLAGS "-g -std=gnu++2a -Werror \
-Wno-error=variadic-macros \
-Werror=implicit-fallthrough")
set(SOURCES ../src/token.cpp
../src/var_substring.cpp
../src/cui.cpp
../src/message_exception.cpp)
add_executable(lexertl_gen.out main_lexertl_gen.cpp ${SOURCES})
Ultimately, I decided to add Travis CI to my project. I wrote a bootstrap.sh script to install and configure all dependencies (I also install GCC 8.1, because default GCC 4.8.4 installed on Travis CI server doesn't support gnu++2a). This script works well, but when it comes to running scripts/build_all.sh on Travis CI server GCC fails to set gnu++2a for compiling my main project with tests, it complains about using inline variables, std::is_integral and other modern C++ stuff, explicitly mentioning that those features are supported under std=c++17 or std=gnu++17. Here is the link to my latest unsuccessful build on Travis CI.
I don't get it, as I updated GCC and my first simple code generator program runs well (though if I remove set(CMAKE_CXX_FLAGS ...) from tests/CMakeLists.txt this program compiles on my PC but doesn't compile on Travis CI server either). What's wrong am I doing here? Is it because of the Cmake 3.9 which Travis uses?
My .travis.yml configuration file:
language: cpp
sudo: required
dist: trusty
compiler:
- g++
os:
- linux
script:
- ./bootstrap.sh; cd scripts; ./build_all.sh; ./run_tests.sh
env:
global:
- LANG="en_US.UTF-8"
matrix:
include:
- os: linux
addons:
apt:
compiler: gcc
sources:
- ubuntu-toolchain-r-test
packages:
- g++-8
env: COMPILER=g++-8
Link to my project on GitHub. (sorry for empty README.md)

Answer 1

Well I've finally solved a problem by making CMAKE_CXX_FLAGS a cache variable:
set(CMAKE_CXX_FLAGS "-g -std=gnu++2a -Werror \
-Wno-error=variadic-macros \
-Werror=implicit-fallthrough"
CACHE STRING "compiler flags")
Though I don't think it is a beautiful solution and I still can't understand why it compiled on my PC but not on Travis CI