Cannot build on Windows for ARM64: error on init_priority attribute.

I tried to compile Boost libraries with `aarch64-w64-mingw32` cross compiler.
Unfortunately, the build process failed after a while with this error message:
```
boost-1.90.0-1.noarch/src/boost-1.90.0/libs/nowide/src/iostream.cpp:148:70: error: 'init_priority' attribute ignored [-Werror=attributes]
148 | #define BOOST_NOWIDE_INIT_PRIORITY __attribute__((init_priority(101)))
| ^
```
At the time of writing, the support for `init_priority` is tested into two CMakeLists.txt files:

- libs/filesystem/CMakeLists.txt
- libs/nowide/CMakeLists.txt

Inside these scripts, `check_cxx_source_compiles()` is used.
These calls check if the test compiles fine and they define `BOOST_NOWIDE_HAS_INIT_PRIORITY` and `BOOST_FILESYSTEM_HAS_INIT_PRIORITY` if everything looks ok.
However, these tests do not fail if there is a just warning.
For example, I tried to compile the test source myself and I got this:
```
$ aarch64-w64-mingw32-g++ boost-1.90.0-1.noarch/src/boost-1.90.0/libs/nowide/config/check_attribute_init_priority.cpp -c -o c.o
boost-1.90.0-1.noarch/src/boost-1.90.0/libs/nowide/config/check_attribute_init_priority.cpp:9:43: warning: 'init_priority' attribute ignored [-Wattributes]
9 | Foo foo __attribute__((init_priority(101)));
| ^
```
So, for CMake the test was good anyways.
The problem comes out because the `-Werror` option, which makes the build process to fail later.

I have not enough knowledge at the moment, but I can see these two solutions:
Adding `-Wno-error=attributes` (if supported) when building those libraries, or
Adding `-Werror` (if supported) when compiling the tests with `check_cxx_source_compiles()`.

This issue happens when building for Windows on ARM64 with GCC.
The `i686-w64-mingw32` and `x86_64-w64-mingw32` cross compilers also work with that attribute, but they don't emit any warning and that's why this issue was not seen before.

I hope that you will find this report useful.

EDIT: it looks like these scripts already handle `CMAKE_REQUIRED_FLAGS` before calling `check_cxx_source_compiles()`:

- libs/atomic/CMakeLists.txt
- libs/log/CMakeLists.txt

Probably, it would be enough to follow the same way for adding the needed option.