Fix build issues when building for RP2040 from pico-examples/pico-sdk.

* include_guard requires GLOBAL as family.cmake is included in multiple non child places
* the following recently added check is suprfluous (family_configure_host_example for rp2040 should do this already),
  and breaks if pico_pio_usb is not avaialble, so i have removed
   # Add pico-pio-usb for rp2040 since user can choose to run on bit-banging host
   if(FAMILY STREQUAL "rp2040")
      family_add_pico_pio_usb(${PROJECT})
   endif()
* added new familt_example_missing_dependency functino to print missing dependency warning, so
  pico-examples can override it to be less in your face, and also more contextual to pico-examples

hw/bsp/family_support.cmake

@@ -1,4 +1,4 @@
-include_guard()
+include_guard(GLOBAL)
 
 include(CMakePrintHelpers)
 
@@ -36,12 +36,14 @@ if (NOT EXISTS ${CMAKE_CURRENT_LIST_DIR}/${FAMILY}/family.cmake)
   message(FATAL_ERROR "Family '${FAMILY}' is not known/supported")
 endif()
 
-# enable LTO if supported
-include(CheckIPOSupported)
-check_ipo_supported(RESULT IPO_SUPPORTED)
-if (IPO_SUPPORTED)
-  set(CMAKE_INTERPROCEDURAL_OPTIMIZATION TRUE)
-endif ()
+if (NOT TINYUSB_OPT_SKIP_CHECK_IPO_SUPPORTED)
+  # enable LTO if supported
+  include(CheckIPOSupported)
+  check_ipo_supported(RESULT IPO_SUPPORTED)
+  if (IPO_SUPPORTED)
+    set(CMAKE_INTERPROCEDURAL_OPTIMIZATION TRUE)
+  endif()
+endif()
 
 set(WARNING_FLAGS_GNU
   -Wall
@@ -278,6 +280,10 @@ function(family_configure_dual_usb_example TARGET RTOS)
   family_configure_example(${TARGET} ${RTOS})
 endfunction()
 
+function(family_example_missing_dependency TARGET DEPENDENCY)
+  message(WARNING "${DEPENDENCY} submodule needed by ${TARGET} not found, please run 'python tools/get_deps.py ${DEPENDENCY}' to fetch it")
+endfunction()
+
 #----------------------------------
 # RPI specific: refactor later
 #----------------------------------