Currently if one package does `depends_on('pkg default_library=shared')`
and another does `depends_on('pkg default_library=both')`, you'd get a
concretization error.
With this PR one package can do `depends_on('pkg default_library=shared')`
and another depends_on('default_library=static'), and it would concretize to
`pkg default_library=shared,static`
Co-authored-by: Massimiliano Culpo <massimiliano.culpo@gmail.com>