Revert C++17 update

This needs to wait until the n-able-Arduino core is updated.

Author: h2zero

builder/frameworks/arduino/nrf5.py

@@ -34,7 +34,7 @@
 env.Append(
     ASFLAGS=["-x", "assembler-with-cpp"],
 
-    CFLAGS=["-std=gnu17"],
+    CFLAGS=["-std=gnu11"],
 
     CCFLAGS=[
         "-Os",  # optimize for size
@@ -50,7 +50,7 @@
     CXXFLAGS=[
         "-fno-rtti",
         "-fno-exceptions",
-        "-std=gnu++17",
+        "-std=gnu++11",
         "-fno-threadsafe-statics"
     ],