Installation issue: cairo can't build with +svg about spack

[teaguesterling]

Digging into this it seems that librsvg is only required to test the SVG functionality, but not required for compilation.

I could think of two ways to potentially address this.

1. Since we also don't have any tests for cairo, I feel like the right course of action here is to remove the librsvg dependency to avoid circular dependencies.
2. Alternatively, I'm not sure if the concretizer will support having two versions of cairo defined in the spec. If so, we could clarify that the librsvg dependency for cairo should be cairo~svg.

I've created a PR for option 1: #44989

[teaguesterling]

Separately, it seems that the librsvg dependencies need to be reviewed whenever it's next version is added, as they don't align with the documentation (here: https://gnome.pages.gitlab.gnome.org/librsvg/devel-docs/compiling.html):

Mandatory dependencies:
Cairo 1.18.0 with PNG support
Freetype2 2.8.0
GIO 2.24.0
Libxml2 2.9.0
Pango 1.46.0

Optional dependencies:
GDK-Pixbuf 2.20.0
GObject-Introspection 0.10.8
gi-docgen
python3-docutils
dav1d 1.3.0 (to support the AVIF image format)

Where we have the following dependencies declared as required:

    depends_on("gobject-introspection", type="build")  # Note: not a variant
...
    depends_on("[email protected]:+gobject")  # Note: no png support required
    depends_on("[email protected]:")  # Note: not a variant