• replace COMPILE_FLAGS with COMPILE_OPTIONS that superceded it.

  Follow-up to 6140dfc
  https://cmake.org/cmake/help/v4.1/prop_sf/COMPILE_FLAGS.html

• replace target_link_libraries() with LINK_FLAGS property for
  CMake <=3.12, because we are passing linker options (not libs).

  Follow-up to 91720b6 cmake: add CURL_CODE_COVERAGE option #18468
  Follow-up to 5488739 cmake: use target_link_options() when available #17670
  Follow-up to 95aea79 CMake: add option to enable Unicode on Windows #5843
  https://cmake.org/cmake/help/v3.7/command/target_link_libraries.html
  https://cmake.org/cmake/help/v3.7/prop_tgt/LINK_FLAGS.html

• replace target_link_options() with LINK_OPTIONS propery for
  CMake 3.13+, to use the modern style.

  Follow-up to 91720b6 cmake: add CURL_CODE_COVERAGE option #18468
  Follow-up to 5488739 cmake: use target_link_options() when available #17670
  https://cmake.org/cmake/help/v3.13/command/target_link_options.html
  https://cmake.org/cmake/help/v3.13/prop_tgt/LINK_OPTIONS.html

Also:

• fix to append to, not override, previously set linker options when
  using CURL_LIBCURL_VERSIONED_SYMBOLS=ON. Before this patch, it was
  overwriting linker options when using CURL_CODE_COVERAGE=ON.

  Follow-up to 91720b6 cmake: add CURL_CODE_COVERAGE option #18468

• consider LINK_FLAGS in place of target_link_libraries() for old cmake

This started out as this modification, before settling to be a simple
tidy-up:

cmake: fix passing compiler options to cmake 3.10 and older

COMPILE_OPTIONS requires 3.11+:
https://cmake.org/cmake/help/v3.19/prop_sf/COMPILE_OPTIONS.html

Replace it with target_compile_options(), and add_compile_options():
https://cmake.org/cmake/help/v3.7/command/target_compile_options.html
https://cmake.org/cmake/help/v3.7/command/add_compile_options.html

Another alternative would be to use COMPILE_FLAGS, but it requires
a space-separated string of options, which is inconvenient:
https://cmake.org/cmake/help/v3.7/prop_sf/COMPILE_FLAGS.html

Also:

• replace COMPILE_FLAGS with modern alternatve when possible,
  and switch to target_compile_options() for old versions.
• replace target_link_options() with LINK_OPTIONS on modern
  cmake.

Follow-up to 91720b6 #18468
Follow-up to 80f9dd0 #17523
Follow-up to e865420 #17047
Follow-up to 45f7cb7 #16238

w/o sp https://github.com/curl/curl/pull/18762/files?w=1

• think it over once again what would be the least bad solution.
  E.g. target_compile_options() could be used for new cmake too,
  but goes against the general direction to use props.
  Perhaps also stick with target_link_options() for 3.13+. [NOT NEEDED]