Build: Add some -Wno-unknown-warning-option flags to CXXFLAGS

Patch from Anonymous.

CMakeLists.txt

@@ -40,7 +40,7 @@ add_custom_target(check-style
     USES_TERMINAL
 )
 
-set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wall -Wextra -Werror -Wmissing-declarations -std=c++2a -fdiagnostics-color=always")
+set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wno-unknown-warning-option -Wall -Wextra -Werror -Wmissing-declarations -std=c++2a -fdiagnostics-color=always")
 if ("${CMAKE_CXX_COMPILER_ID}" STREQUAL "GNU")
     set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fconcepts")
 elseif ("${CMAKE_CXX_COMPILER_ID}" STREQUAL "Clang")

Kernel/CMakeLists.txt

@@ -239,7 +239,7 @@ set(SOURCES
     ${CRYPTO_SOURCES}
 )
 
-set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -DKERNEL")
+set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wno-unknown-warning-option -DKERNEL")
 set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -pie -fPIE -ffreestanding -fbuiltin")
 set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -mno-80387 -mno-mmx -mno-sse -mno-sse2")
 set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fno-asynchronous-unwind-tables")

Libraries/LibC/CMakeLists.txt

@@ -52,7 +52,7 @@ file(GLOB AK_SOURCES "../../AK/*.cpp")
 file(GLOB ELF_SOURCES "../LibELF/*.cpp")
 set(ELF_SOURCES ${ELF_SOURCES} ../LibELF/Arch/i386/plt_trampoline.S)
 
-set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -DSERENITY_LIBC_BUILD")
+set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wno-unknown-warning-option -DSERENITY_LIBC_BUILD")
 
 find_program(INSTALL_COMMAND ginstall)
 if(NOT INSTALL_COMMAND)

Meta/Lagom/CMakeLists.txt

@@ -1,7 +1,7 @@
 cmake_minimum_required (VERSION 3.0)
 project (Lagom)
 
-set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -O2 -Wall -Wextra -Werror -std=c++2a -fPIC -g -Wno-deprecated-copy")
+set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wno-unknown-warning-option -O2 -Wall -Wextra -Werror -std=c++2a -fPIC -g -Wno-deprecated-copy")
 
 if ("${CMAKE_CXX_COMPILER_ID}" STREQUAL "Clang")
     set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wconsumed -Wno-overloaded-virtual")