Install MUSE_CPPFLAGS
A new environmental, MUSE_CPPFLAGS can now be set by writing in the .muse file:
CPPFLAGS -Wno-deprecated-declarations
or by setting it in the envset. Depending on if the need is generated in the code or in the dependencies.
This will allow a smoother transition to different compiler switches.
We need this one above, now, in order to accommodate BTrk and e28, but when BTrk is retired, we can remove this flag from .muse, while it will continue to be included in builds of older code.