I created a PR which modifies all compiler packages by adding
-D__MYNEWT__=1 to compiler.flags.base, see here:
The reason for this is to have some common symbol which can be used to
distinguish between building code for Mynewt or for some other platform -
similar symbols are defined for other platforms. This will be especially
useful for components which have separate repository, e.g. mynewt-nffs or
upcoming mynewt-nimble which can be also ported to other platforms so may
use different set of headers or some porting layer.