build: prepare builds for clang-cl, add cmake ossfuzz support

- cmake: add support to build ossfuzz.
  Enable with `-DBUILD_OSSFUZZ=ON`.
  Also supports `-DLIB_FUZZING_ENGINE=` like autotools does.
- check for `__clang__` when suppressing warnings in source. Necessary
  for clang-cl, which set `__clang__`, but doesn't set `__GNU__`.
- cmake: optimize out 4 picky warning option detections with gcc.
- cmake: bring `-pedantic-error`, `-Wall` use closer to curl's.
- cmake: set `-Wno-language-extension-token` for clang-cl.
- cmake: escape only the necessary `-W` options for clang-cl.
- cmake: apply picky warnings to C++.
- cmake: replace `unset(VAR)` with `set(VAR "")` for init.
- cmake: prefer dash-style MSVC options.
- cmake: simplify `MATCHES` expression.
- cmake: formatting/whitespace.
- ci/GHA: bump `actions/upload-artifact` to v4

Closes #1524

.github/workflows/cifuzz.yml

@@ -31,7 +31,7 @@ jobs:
         dry-run: false
         language: c
     - name: Upload Crash
-      uses: actions/upload-artifact@v3
+      uses: actions/upload-artifact@v4
       if: ${{ failure() && steps.build.outcome == 'success' }}
       with:
         name: artifacts