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

example/tcpip-forward.c

@@ -246,12 +246,12 @@ int main(int argc, char *argv[])
     for(;;) {
         fd_set fds;
         FD_ZERO(&fds);
-#if defined(__GNUC__)
+#if defined(__GNUC__) || defined(__clang__)
 #pragma GCC diagnostic push
 #pragma GCC diagnostic ignored "-Wsign-conversion"
 #endif
         FD_SET(forwardsock, &fds);
-#if defined(__GNUC__)
+#if defined(__GNUC__) || defined(__clang__)
 #pragma GCC diagnostic pop
 #endif
         tv.tv_sec = 0;
@@ -261,12 +261,12 @@ int main(int argc, char *argv[])
             fprintf(stderr, "failed to select().\n");
             goto shutdown;
         }
-#if defined(__GNUC__)
+#if defined(__GNUC__) || defined(__clang__)
 #pragma GCC diagnostic push
 #pragma GCC diagnostic ignored "-Wsign-conversion"
 #endif
         if(rc && FD_ISSET(forwardsock, &fds)) {
-#if defined(__GNUC__)
+#if defined(__GNUC__) || defined(__clang__)
 #pragma GCC diagnostic pop
 #endif
             ssize_t nwritten;