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

tests/ossfuzz/CMakeLists.txt

@@ -0,0 +1,24 @@
+# Copyright (C) Viktor Szakats
+# SPDX-License-Identifier: BSD-3-Clause
+
+enable_language(CXX)
+
+add_library(standaloneengine STATIC "standaloneengine.cc")
+target_include_directories(standaloneengine PRIVATE
+  "${PROJECT_SOURCE_DIR}/include")
+set_target_properties(standaloneengine PROPERTIES UNITY_BUILD OFF)
+
+add_executable(ssh2_client_fuzzer "ssh2_client_fuzzer.cc" "testinput.h")
+target_include_directories(ssh2_client_fuzzer PRIVATE
+  "${PROJECT_SOURCE_DIR}/include")
+if(LIB_FUZZING_ENGINE)
+  if(LIB_FUZZING_ENGINE STREQUAL "-fsanitize=fuzzer")  # FIXME: compiler-specific
+    set_target_properties(ssh2_client_fuzzer PROPERTIES LINK_FLAGS "${LIB_FUZZING_ENGINE}")
+  else()
+    target_link_libraries(ssh2_client_fuzzer ${LIB_FUZZING_ENGINE})
+  endif()
+else()
+  target_link_libraries(ssh2_client_fuzzer standaloneengine)
+endif()
+target_link_libraries(ssh2_client_fuzzer ${LIB_STATIC} ${LIBSSH2_LIBS_SOCKET})
+set_target_properties(ssh2_client_fuzzer PROPERTIES UNITY_BUILD OFF)

tests/ossfuzz/Makefile.am

@@ -31,4 +31,4 @@ ssh2_client_fuzzer_LDFLAGS = $(AM_LDFLAGS) -static
 libstandaloneengine_a_SOURCES = standaloneengine.cc
 libstandaloneengine_a_CXXFLAGS = $(AM_CXXFLAGS)
 
-EXTRA_DIST = ossfuzz.sh
+EXTRA_DIST = CMakeLists.txt ossfuzz.sh