License is not clear

[SMillerDev]

Describe the bug
The license terms are not clear, the license is GPLv3, which could allow any later version too if the files specify that.
The source files specify that the license is BSD-2 though, so what is the license of all files in this repo?
And if it is the way the License file specifies, rather than the way the source specifies. Can we use a later version too?

To Reproduce
Steps to reproduce the behavior:
Compare https://github.com/mellowcandle/bitwise/blob/master/src/stack.c and https://github.com/mellowcandle/bitwise/blob/master/LICENSE

Expected behavior
A defined license in both LICENSE and source files

Version:

• master

Desktop (please complete the following information):

• OS: any

Additional context
Homebrew/homebrew-core#137210 reports usage of an outdated license identifier and I can't figure out what it should be either

[rfried-nrl]

Please see debian/copyright some files are gpl and some are bsd2. This is valid

[SMillerDev]

I'm not saying it's invalid, I'm saying it's unclear. Because that stack.c file refers to the LICENSE file, which does not contain a BSD license.