slot_map : Fix gcc7 structured bindings warnings.

Gcc7 doesn't implement [[maybe_unused]] attribute for structured
bindings. Disable unused-parameter checks on gcc7 (only).

slot_map.h:48:91: error: unused variable ¿gen¿ [-Werror=unused-variable]
     static constexpr auto get_index(const Key& k) { [[maybe_unused]] const auto& [idx, gen] = k; return idx; }

Also disable buggy unused-but-set-variable for gcc7 (only).

error: variable ¿idx¿ set but not used [-Werror=unused-but-set-variable]
  template<class Integral> static constexpr void set_index(Key& k, Integral value) { auto&[idx, gen] = k; detail::maybe_unused(gen); idx = static_cast<key_size_type>(value); }

CMakeLists.txt

@@ -70,6 +70,11 @@ elseif ("${CMAKE_CXX_COMPILER_ID}" STREQUAL "GNU")
 	set_source_files_properties(${SG14_TEST_SOURCE_DIRECTORY}/plf_colony_test.cpp PROPERTIES
 		COMPILE_FLAGS "-Wno-unused-parameter"
 	)
+	if (CMAKE_CXX_COMPILER_VERSION MATCHES "^7.*")
+		set_source_files_properties(${SG14_TEST_SOURCE_DIRECTORY}/slot_map_test.cpp PROPERTIES
+			COMPILE_FLAGS "-Wno-error=unused-variable -Wno-error=unused-but-set-variable") # Fix gcc7 issues with structured bindings
+		message("Disabled -Wunused-variable and -Wunused-but-set-variable for gcc ${CMAKE_CXX_COMPILER_VERSION}.")
+	endif()
 elseif ("${CMAKE_CXX_COMPILER_ID}" STREQUAL "MSVC")
 	set_property(DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR} PROPERTY VS_STARTUP_PROJECT ${TEST_NAME})
 	target_compile_options(${TEST_NAME} PRIVATE /Zc:__cplusplus /permissive- /W4 /WX)