Fix failing compilation tests in g++11

Request `-std=c++11` explicitly in tests meant to compile against that
standard.

CMakeLists.txt

@@ -16,6 +16,10 @@ set(CMAKE_REQUIRED_FLAGS "-std=c++1z") # only for this check
 CHECK_CXX_SYMBOL_EXISTS(__cpp_noexcept_function_type "" HAS_NOEXCEPT_IN_TYPE)
 unset(CMAKE_REQUIRED_FLAGS)
 
+# request C++11 explicitly by default - works around newer compilers otherwise
+# omitting `-std=c++11` despite 11 being requested with target_compile_features
+set(CMAKE_CXX_STANDARD 11)
+
 # compiler warnings
 if("${CMAKE_CXX_COMPILER_ID}" STREQUAL "Clang")
   set(clang_warnings "-Weverything -pedantic -Wno-c++98-compat \