make: export RIOT_VERSION to CFLAGS_WITH_MACROS

All macros are filtered out of the `CFLAGS` variable and during this
process, the `CFLAGS` string is destroyed. This is due to the fact that
the `RIOT_VERSION` string contains spaces, but the filtering process does
not take the whole string into account (following the first space). This
leads to some parts of the `RIOT_VERSION` string be sill existent in the
`CFLAGS` string.

Instead of exporting the `RIOT_VERSION` string to the `CFLAGS` macro to
just remove it again from there, I exported it to `CFLAGS_WITH_MACROS`.

Fixes #5859