> Martin, can you please elaborate on this? I never heard of such
> "standards" in OSS.

MAL already gave the link. From the link:

Sometimes package developers are tempted to set user variables such as
CFLAGS because it appears to make their job easier. However, the package
itself should never set a user variable, particularly not to include
switches that are required for proper compilation of the package. Since
these variables are documented as being for the package builder, that
person rightfully expects to be able to override any of these variables
at build time.