fix MinGW linker error

azure-pipelines.yml

@@ -122,7 +122,8 @@ jobs:
           # conan install .. -s build_type=Release --profile ../.ci/conan_profile_mingw_gcc8 --build=missing
           # build in CI with gcc 7 and libc++ as conan MinGW doesn't build variants with libstdc++11 for now
           # and no gcc 8 build are available, see also - https://github.com/conan-io/conan-package-tools/issues/375
-          conan install .. -s build_type=Release --profile ../.ci/conan_profile_mingw_gcc8 -s compiler.version=7 -s compiler.libcxx=libstdc++ --build=missing
+          set CONAN_ENV_COMPILER_LIBCXX=libstdc++
+          conan install .. -s build_type=Release --profile ../.ci/conan_profile_mingw_gcc8 -s compiler.version=7 --build=missing
           call .\activate_run.bat
           # see https://stackoverflow.com/a/45104058 - MinGW sh.exe must NOT be in your path
           cmake -G "MinGW Makefiles" -DCMAKE_BUILD_TYPE=Release -DCMAKE_SH=CMAKE_SH-NOTFOUND ..