It is important to note that
make.conf is not reliable at all, to check how a package is compiled. In fact, it is just one of the supplementary config files in a whole list.
USE_ORDER = "env:pkg:conf:defaults:pkginternal:repo:env.d"
Determines the precedence of layers in the incremental stacking of the USE
variable. Precedence decreases from left to right such that env overrides
pkg, pkg overrides conf, and so forth.
So the priority is handled as:
- Shell environment when calling emerge
- Package specific USE setting in /etc/portage/package.use
- Defaults from selected profile
- Defaults set in package
- Distribution defaults
- Settings defined in /etc/env.d
So basically an USE flags can be set without being mentioned in
make.conf. Likewise for CFLAGS (although their order of processing is fixed, but it works similar)
Likewise, USE flags can be masked in a profile without any in the ebuild or any other sources mentioned above. In short, based on config files you can't.
If you want to know the resulting global settings currently used by the system, use
emerge --info. This should give you a massive amount of details. Usually CFLAGS are not very often overridden for a single package,so you can consider that output definitive.
If you want to know the USE flags of a specific package that is installed, you could use
equery u <package>, provided the gentoolkit package is installed.