This file seems to not do anything useful. From what I can tell the
OSX_FATLIB option sets CMAKE_OSX_ARCHITECTURES to "x86_64;i386". This
doesn't account for the arm that apple now has, as well as 32 bit
support being completely removed, and I think it's entirely reasonable
to expect users to pass the necessary architectures they want
themselves. It's possible this could break some users who rely on this,
but I sincerely doubt anyone does. The solution is trivial either way
(put -DCMAKE_OSX_ARCHITECTURES="i386;x86_64" on the command line
yourself)

The second part of BuildOptions.cmake claims to set PLATFORM to "Web" if
the emscripten toolchain file is used (if (EMSCRIPTEN)), but it does not
work correctly anyway. Currently, glfw searches for wayland and x11
libraries and fails likeso:

CMake Error at /usr/share/cmake/Modules/FindPkgConfig.cmake:645 (message):
  The following required packages were not found:
   - wayland-client>=0.2.7
   - wayland-cursor>=0.2.7
   - wayland-egl>=0.2.7
   - xkbcommon>=0.5.0

Call Stack (most recent call first):
  /usr/share/cmake/Modules/FindPkgConfig.cmake:873 (_pkg_check_modules_internal)
  src/external/glfw/src/CMakeLists.txt:163 (pkg_check_modules)

Considering this code doesn't work as described, it's okay to delete it.
I think a better check should be implemented, but that is for a
different PR.