Check for minimum C++ standard version doesn't work

[Mi-La]

Zserio version and language
Zserio: 2.14.0
Language: C++

Describe the bug
Check for minimum C++ standard doesn't work in some cases.

Expected behavior
The check will fire an error when C++ standard is lower than C++11.

Additional context
The check is here

zserio/compiler/extensions/cpp/runtime/src/CMakeLists.txt

Lines 20 to 22 in aba07d1

	if ("${CMAKE_CXX_STANDARD}" STREQUAL "98")
	message(FATAL_ERROR "C++98 standard is not supported, please set CMAKE_CXX_STANDARD at least to C++11!")
	endif ()

.

It seems that when compilers uses a default C++ standard version which is lower than C++11, the check doesn't work.

See failed builds on conan-center-index for MacOS Clang 13 and gcc 5.

[Mi-La]

See conan-io/conan-center-index#23846 (comment).