cmake: Treat warnings as errors on Clang and GCC

-Wformat-security is enabled, some Linux packages build system have it
enabled.
obsproject#5766

-Wunused-parameter is enabled too.

Also ensure that null conversion (C++ only for GCC) is enabled because its
enablement by default can depend on the CMake generator.

-Wno-error=deprecated-declarations is added on Linux/FreeBSD for release
builds to help packager.

cmake/Modules/CompilerConfig.cmake

@@ -91,17 +91,24 @@ else()
   endif()
 
   add_compile_options(
+    -Werror
     -Wextra
     -Wvla
     -Wswitch
+    -Wformat
+    -Wformat-security
+    -Wunused-parameter
     -Wno-unused-function
     -Wno-missing-field-initializers
     -fno-strict-aliasing
     "$<$<COMPILE_LANGUAGE:C>:-Werror-implicit-function-declaration;-Wno-missing-braces>"
     "$<$<BOOL:${USE_LIBCXX}>:-stdlib=libc++>"
     "$<$<CONFIG:DEBUG>:-DDEBUG=1;-D_DEBUG=1>"
-    "$<$<COMPILE_LANG_AND_ID:CXX,AppleClang,Clang>:-fcolor-diagnostics>"
-    "$<$<COMPILE_LANG_AND_ID:C,AppleClang,Clang>:-fcolor-diagnostics>")
+    "$<$<COMPILE_LANG_AND_ID:CXX,AppleClang,Clang>:-Wnull-conversion;-fcolor-diagnostics>"
+    "$<$<COMPILE_LANG_AND_ID:C,AppleClang,Clang>:-Wnull-conversion;-fcolor-diagnostics>"
+    "$<$<COMPILE_LANG_AND_ID:CXX,GNU>:-Wconversion-null>"
+    "$<$<AND:$<BOOL:${OS_POSIX}>,$<NOT:$<BOOL:${OS_MACOS}>>,$<NOT:$<CONFIG:DEBUG>>>:-Wno-error=deprecated-declarations>"
+  )
 
   if(OBS_CODESIGN_LINKER)
     add_link_options("LINKER:$<$<PLATFORM_ID:Darwin>:-adhoc_codesign>")