Make `-Wnoexcept-type` non-fatal, if present

This warning was introduced with gcc-7 to (quoting the documentation)
"Warn if the C++1z feature making noexcept part of a function type
changes the mangled name of a symbol relative to C++14. Enabled by -Wabi
and -Wc++1z-compat.".

It is triggered from the `GenericFunctor` class template. It can be
safely made not fatal, because this class template is not exposed
outside and the whole project is currently compiled with the same C++
standard compiler flags.

This commit adds machinery to `CMakeLists.txt` to make the warning not
fatal, but only if present. Disabling it when not present would trigger
build errors.

CMakeLists.txt

@@ -55,6 +55,13 @@ include_directories("${QEMU_INSTALL_PATH}/include/")
 set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11 -Wall -Werror -Wno-error=unused-variable")
 set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wno-error=return-type")
 set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wno-error=unused-function")
+
+include(CheckCXXCompilerFlag)
+CHECK_CXX_COMPILER_FLAG("-Wno-error=noexcept-type" COMPILER_SUPPORTS_WARN_NOEXCEPT_TYPE)
+if (COMPILER_SUPPORTS_WARN_NOEXCEPT_TYPE)
+  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wno-error=noexcept-type")
+endif()
+
 if(CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
   set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wno-error=unused-but-set-variable")
   set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -Wno-error=maybe-uninitialized")