posix: define M_PI and other stuff in Zephyr sketches

Define _XOPEN_SOURCE=700 to use POSIX 2017 features in the loader and
Zephyr sketches.

Author: pillo79

loader/CMakeLists.txt

@@ -23,5 +23,6 @@ target_sources_ifdef(CONFIG_USB_DEVICE_STACK_NEXT app PRIVATE
 
 FILE(GLOB app_sources *.c)
 target_sources(app PRIVATE ${app_sources})
+target_compile_definitions(app PUBLIC _XOPEN_SOURCE=700)
 
 target_link_libraries(app PUBLIC blobs)