Relax warnings in cmake files

[c0nstexpr]

https://github.com/boost-ext/ut/blob/a0311a0b1a7bcab7ad7aece602c7d3db07197bc9/CMakeLists.txt#L69 https://github.com/boost-ext/ut/blob/a0311a0b1a7bcab7ad7aece602c7d3db07197bc9/CMakeLists.txt#L70 Add extra warning compile makes other some project compilation failed, if the project treat warning as errors. The following shows the compile options:

{ "directory": "...", "command": "...\clang++.exe -DFMT_LOCALE -DFMT_SHARED -Istdsharp/tests/include -Istdsharp/include -isystem stdsharp/build/Clang64-Debug/_deps/boost-ext-ut-src/include -isystem stdsharp/build/Clang64-Debug/vcpkg_installed/x64-mingw-dynamic/include -m64 -g -Wno-logical-op-parentheses -Wno-dangling-else -Werror -Wa,-mbig-obj -Wall -Wextra -Wno-c++98-c++11-c++14-c++17-compat-pedantic -Wno-c++98-c++11-compat -Wno-c++98-compat -Wno-c++98-compat-pedantic -Wno-c99-extensions -Wno-pedantic -std=gnu++2b -o tests\CMakeFiles\stdsharpTests_LIB.dir\src\functional\decompose_test.cpp.obj -c stdsharp\tests\src\functional\decompose_test.cpp", "file": "stdsharp\tests\src\functional\decompose_test.cpp" },

Errors:

error: add explicit braces to avoid dangling else [-Werror,-Wdangling-else]

But '-Wno-dangling-else' was already specified.