Attempted fix for builds failing with compilers older than GCC 6

CMakeLists.txt

@@ -57,7 +57,7 @@ option(ENABLE_HISTORY "Enable using GNU history for history in lua console" ON)
 
 if(NOT CMAKE_CROSSCOMPILING)
 	try_run(IEEE754_TEST_RETURN_CODE IEEE754_TEST_COMPILED ${CMAKE_BINARY_DIR} ${CMAKE_SOURCE_DIR}/src/compile_time_tests/ieee_754.cpp
-		OUTPUT_VARIABLE IEEE754_TEST_OUTPUT)
+		CMAKE_FLAGS "-DCXX_STANDARD=11" OUTPUT_VARIABLE IEEE754_TEST_OUTPUT)
 	if(NOT IEEE754_TEST_RETURN_CODE EQUAL 0)
 		message(${IEEE754_TEST_OUTPUT})
 		message(FATAL_ERROR "Your platform does not represent floating point numbers in the IEEE 754 format.")

SConstruct

@@ -319,6 +319,10 @@ if env["multilib_arch"]:
 # Some tests need to load parts of boost
 env.PrependENVPath('LD_LIBRARY_PATH', env["boostlibdir"])
 
+# Some tests require at least C++11
+env.AppendUnique(CCFLAGS = Split("-W -Wall"), CFLAGS = ["-std=c99"])
+env.AppendUnique(CXXFLAGS = "-std=c++" + env["cxx_std"])
+
 if env["prereqs"]:
     conf = env.Configure(**configure_args)
 
@@ -463,10 +467,6 @@ for env in [test_env, client_env, env]:
     env.Append(CPPDEFINES = ["HAVE_CONFIG_H"])
 
     if "gcc" in env["TOOLS"]:
-        env.AppendUnique(CCFLAGS = Split("-W -Wall"), CFLAGS = ["-std=c99"])
-
-        env.AppendUnique(CXXFLAGS = "-std=c++" + env["cxx_std"])
-
         if env['openmp']:
             env.AppendUnique(CXXFLAGS = ["-fopenmp"], LIBS = ["gomp"])