Specifying compiler versions in travis for cmake builds

The travis Building a C++ Project documentation shows how to specify gcc and clang compiler versions in build matrices. However, it does not show how to build projects with those compilers using cmake.

I amended the .travis.yml file here to specify gcc 9 and clang 8 as per the travis documentation, i.e.:

matrix:  include:    - compiler: gcc      addons:        apt:          sources:            - ubuntu-toolchain-r-test          packages:            - g++-9      env:        - MATRIX_EVAL="CC=gcc-9 && CXX=g++-9"    - compiler: clang      addons:        apt:          sources:            - ubuntu-toolchain-r-test            - llvm-toolchain-bionic-8          packages:            - clang-8            - libstdc++-8-dev      env:        - MATRIX_EVAL="CC=clang-8 && CXX=clang++-8"before_install:  - eval "${MATRIX_EVAL}"  - pip install --user cpp-coveralls...script:  - mkdir _builds  - cd _builds  - cmake -DVIA_HTTPLIB_UNIT_TESTS=ON -DVIA_HTTPLIB_COVERAGE=ON ${CMAKE_OPTIONS} ..   - make  - ./via-httplib_test

But is caused build errors when running cmake, e.g.:

$ cmake -DVIA_HTTPLIB_UNIT_TESTS=ON -DVIA_HTTPLIB_COVERAGE=ON ${CMAKE_OPTIONS} ..CMake Error at /usr/local/cmake-3.12.4/share/cmake-3.12/Modules/CMakeDetermineCCompiler.cmake:48 (message):  Could not find compiler set in environment variable CC:  gcc-9.Call Stack (most recent call first):  CMakeLists.txt:9 (project)CMake Error: CMAKE_C_COMPILER not set, after EnableLanguageCMake Error: CMAKE_CXX_COMPILER not set, after EnableLanguage-- Configuring incomplete, errors occurred!See also "/home/travis/build/kenba/via-httplib/_builds/CMakeFiles/CMakeOutput.log".The command "cmake -DVIA_HTTPLIB_UNIT_TESTS=ON -DVIA_HTTPLIB_COVERAGE=ON ${CMAKE_OPTIONS} .." exited with 1.

I tried fixing the errors by specifying CMAKE_C_COMPILER and CMAKE_CXX_COMPILER for cmake, but I could not get it to work. However, it builds correctly with:

  env:    - MATRIX_EVAL="CC=gcc && CXX=g++"  ...  env:    - MATRIX_EVAL="CC=clang && CXX=clang++"

but builds with the default bionicgcc and clang compilers, i.e.: GCC 7.4.0 and Clang 7, not GCC 9 and Clang 8.

How to write a .travis.yml file so that cmake can find and use the compiler versions specified in a matrix?