m4/ax_cxx_compile_stdcxx.m4: workarounds to allow gcc 4.8

gcc 4.8 doesn't have full C++14 support, but it's enough for MPD.

m4/ax_cxx_compile_stdcxx.m4

@@ -449,7 +449,7 @@ m4_define([_AX_CXX_COMPILE_STDCXX_testbody_new_in_14], [[
 
 #error "This is not a C++ compiler"
 
-#elif __cplusplus < 201402L
+#elif __cplusplus < 201300L
 
 #error "This is not a C++14 compiler"
 
@@ -484,6 +484,7 @@ namespace cxx14
 
   }
 
+#ifdef DISALLOW_GCC48
   namespace test_generalized_constexpr
   {
 
@@ -503,6 +504,7 @@ namespace cxx14
     static_assert(strlen_c("another\0test") == 7UL, "");
 
   }
+#endif
 
   namespace test_lambda_init_capture
   {