Add check for pmr default_ressource related functions

Some versions of `libc++` on Apple are missing the implementation of the `pmr::get_default_resource()`. Some Boost.GIL tests are impacted and discussed in https://github.com/boostorg/gil/pull/778.

In CMake terms, the check would look something like this:

```
check_cxx_source_compiles( "
#include <${pmr_include}>
int main() {
${pmr_ns}::set_default_resource(nullptr);
${pmr_ns}::memory_resource* mr = ${pmr_ns}::get_default_resource();
return 0;
}
"BOOST_HAVE_DEFAULT_RESOURCE)
```

A polyfill would be easy to add for the missing symbols.