HEADER_GLOBAL was using non-standard attributes to achieve what C++17
now permits with a keyword. Use the standard formulation.
Update the README to note that gcc is still not recommended, but because
of its poor codegen for 128-bit atomic compare and exchange, rather than
because it doesn't support the attribute used for HEADER_GLOBAL.
Make header only library target.
Use the CMake INTERFACE target type to include the build settings for
snmalloc in other projects using headers only.
Made warnings setting a macro for reuse.
