When NEWLIB_INCLUDE_DIR is set from other parts than 'COMPILER_INCLUDE_PATHS' it
does not have a trailing slash.
Also, it makes it more problematic when supplying it from the command line.
And anyway having two '/' does not break anything.
In the previous state, with llvm and arm for example, newlib-nano include dir
NEWLIB_NANO_INCLUDE_DIR is placed after NEWLIB_INCLUDES and so the default
'newlib.h' is used instead of the nano version.
