SCons: Explicitly define our C (C11) and C++ (C++14) standards

On GCC and Clang, we use C11 and C++14 with GNU extensions (`std=gnu11`
and `std=gnu++14`). Those are the defaults for current GCC and Clang,
and also match the feature sets we want to use in Godot.

On MSVC, we require C++14 support explicitly with `/std:c++14`, and
make it strict with the use of `/permissive-` (so features of C++17 or
later can't be used).

Moves the definition before querying environment flags and platform
config so that it can be overridden when necessary.

SConstruct

@@ -292,6 +292,7 @@ if selected_platform in platform_list:
     if env["extra_suffix"] != '':
         env.extra_suffix += '.' + env["extra_suffix"]
 
+    # Environment flags
     CCFLAGS = env.get('CCFLAGS', '')
     env['CCFLAGS'] = ''
     env.Append(CCFLAGS=str(CCFLAGS).split())
@@ -308,18 +309,28 @@ if selected_platform in platform_list:
     env['LINKFLAGS'] = ''
     env.Append(LINKFLAGS=str(LINKFLAGS).split())
 
+    # Set our C and C++ standard requirements.
+    # Prepending to make it possible to override
+    if not env.msvc:
+        # Specifying GNU extensions support explicitly, which are supported by
+        # both GCC and Clang. This mirrors GCC and Clang's current default
+        # compile flags if no -std is specified.
+        env.Prepend(CFLAGS=['-std=gnu11'])
+        env.Prepend(CXXFLAGS=['-std=gnu++14'])
+    else:
+        # MSVC doesn't have clear C standard support, /std only covers C++.
+        # We apply it to CCFLAGS (both C and C++ code) in case it impacts C features.
+        env.Prepend(CCFLAGS=['/std:c++14', '/permissive-'])
+
+    # Platform specific flags
     flag_list = platform_flags[selected_platform]
     for f in flag_list:
         if not (f[0] in ARGUMENTS):  # allow command line to override platform flags
             env[f[0]] = f[1]
 
-    # must happen after the flags, so when flags are used by configure, stuff happens (ie, ssl on x11)
+    # Must happen after the flags definition, so that they can be used by platform detect
     detect.configure(env)
 
-    # Enable C++11 support
-    if not env.msvc:
-        env.Append(CXXFLAGS=['-std=c++11'])
-
     # Configure compiler warnings
     if env.msvc:
         # Truncations, narrowing conversions, signed/unsigned comparisons...