zephyr_library_compile_options(): silently de-duplicate

PR #14776 / commit 915a3532 changed function
zephyr_library_compile_options() to use MD5 instead of RANDOM for build
reproduction reasons. As later predicted by Sebastian Bøe in the PR
(thx), the names generated for these pseudo-libraries won't be unique
anymore if the exact same flag gets added more than once.

To reproduce the issue, simply add the following two lines in some
CMakeLists.txt file like samples/hello_world/CMakeLists.txt:

zephyr_library_compile_options("-Dfubar=barfu")
zephyr_library_compile_options("-Dfubar=barfu")

cmake will then fail like this:

 CMake Error at zephyr/cmake/extensions.cmake:403 (add_library):
  add_library cannot create target
  "options_interface_lib_e689fdb7eb15d801c2f95dd61301fc5e" because
  another target with the same name already exists.  The existing
  target is an interface library created in source directory
  "zephyr/samples/hello_world".  See documentation for
  policy CMP0002 for more details.
 Call Stack (most recent call first):
  CMakeLists.txt:9 (zephyr_library_compile_options)

As requested by Sebastian, silently discard duplicate options just like
CMake does.

Fixes #15093

Signed-off-by: Marc Herbert <marc.herbert@intel.com>