cmake: no -Werror-implicit-function-declaration for C++

This flag was ignored for C++. Starting with GCC-9 this
flag is complained about.

CMakeLists.txt

@@ -82,7 +82,7 @@ if(CMAKE_SYSTEM_PROCESSOR MATCHES "^(powerpc|ppc)64le")
 endif()
 
 if(CMAKE_COMPILER_IS_GNUCC OR CMAKE_COMPILER_IS_GNUCXX OR CMAKE_COMPILER_IS_CLANG)
-	set(CMAKE_CXX_FLAGS "-Wall -Wextra -Wvla -Wno-unused-function -Werror-implicit-function-declaration -Wno-missing-field-initializers ${CMAKE_CXX_FLAGS} -fno-strict-aliasing")
+	set(CMAKE_CXX_FLAGS "-Wall -Wextra -Wvla -Wno-unused-function -Wno-missing-field-initializers ${CMAKE_CXX_FLAGS} -fno-strict-aliasing")
 	set(CMAKE_C_FLAGS "-Wall -Wextra -Wvla -Wno-unused-function -Werror-implicit-function-declaration -Wno-missing-braces -Wno-missing-field-initializers ${CMAKE_C_FLAGS} -std=gnu99 -fno-strict-aliasing")
 
 	option(USE_LIBC++ "Use libc++ instead of libstdc++" ${APPLE})