Changes:
- Provides a workaround for
  [emilk#249](emilk#249)
- Adds an option that can be used to disable compile warnings for
  the loguru library. This could be useful for users who just want to
  build the library and dont care about the warnings.
  (or they just aren't in a position to fix them anyways).

Notes:
- The default behaviour remains the same: compiler-warnings are enabled
  by default when the project is built as a top-level project, and
  disabled by default when the project is included as a sub-project with
  either add_subdirectory() or FetchContent()

Notes:
- This is needed to quiet a cmake warning about non-cache variables
  being used to set loguru cmake `options()`, because loguru's minimum
  cmake version is still 3.10 and this wasn't allowed until cmake 3.13.

  ```
  # CMakeLists.txt

  # prior to cmake 3.13 the user would need to set options as cache vars
  set(LOGURU_ENABLE_COMPILER_WARNINGS FALSE CACHE BOOL "" FORCE)

  # cmake 3.13 and onwards the sub-project options respect normal vars.
  set(LOGURU_ENABLE_COMPILER_WARNINGS FALSE)

  add_subdirectory(loguru)
  ```

Changes:
- Provides a workaround for [emilk#249](emilk#249)
- Removed `-Werror` from the default compiler warnings for GCC.
- Added an option that can be used to opt-in to warnings-as-errors for
  all three supported compilers (gcc, clang, msvc) if desired.

Notes:
- Realized I must've dropped the actual most important commit while
  rebasing this branch before submission, sheesh