Make examples friendlier for non GCC compilers and fixed slave_mem_i2c.c to compile on boards without I2C pins (#336)

CMakeLists.txt

@@ -28,8 +28,10 @@ add_subdirectory(hello_world)
 add_compile_options(-Wall
         -Wno-format          # int != int32_t as far as the compiler is concerned because gcc has int32_t as long int
         -Wno-unused-function # we have some for the docs that aren't called
-        -Wno-maybe-uninitialized
         )
+if (CMAKE_C_COMPILER_ID STREQUAL "GNU")
+    add_compile_options(-Wno-maybe-uninitialized)
+endif()
 
 # Hardware-specific examples in subdirectories:
 add_subdirectory(adc)