cmake: do all compile tests with -D_GNU_SOURCE=1

CMakeLists.txt

@@ -336,6 +336,8 @@ if(WINDOWS)
   set(CMAKE_SHARED_LIBRARY_PREFIX "")
 endif()
 
+set(CMAKE_REQUIRED_FLAGS "${CMAKE_REQUIRED_FLAGS} -D_GNU_SOURCE=1")
+
 # Emscripten toolchain has a nonempty default value for this, and the checks
 # in this file need to change that, so remember the original value, and
 # restore back to that afterwards. For check_function_exists() to work in