stellarium: Don't use exiv2; do use eigen3 and glm

[ryandesign]

Description

This PR changes two things:

1. Don't use exiv2 even if it is found. It is searched for by the LensDistortionEstimator plugin but it appears to be optional. If it were found, it would fail to build, because it requires a newer version of exiv2 than we currently have in MacPorts.
2. Do depend on eigen3 and glm. The ShowMySky pluing looks for these, and if it doesn't find them, it appears to use its own internal copies:

-- CPM: Adding package ShowMySky-Qt5@0.3.1 (0.3.1)
-- Looking for C++ include glm/glm.hpp
-- Looking for C++ include glm/glm.hpp - not found
-- CPM: ShowMySky-Qt5: Adding package glm@ ()
-- CPM: ShowMySky-Qt5: Adding package Eigen3@3.4.0 (3.4.0)
-- ABI version: 15
-- Will build ShowMySky library

After adding the dependencies, it uses MacPorts versions:

-- CPM: Adding package ShowMySky-Qt5@0.3.1 (0.3.1)
-- Looking for C++ include glm/glm.hpp
-- Looking for C++ include glm/glm.hpp - found
-- Checking that GLM has the required features
-- Checking that GLM has the required features - done
-- CPM: ShowMySky-Qt5: Using local package Eigen3@3.4.0
-- ABI version: 15
-- Will build ShowMySky library

Type(s)

• bugfix
• enhancement
• security fix

Tested on

macOS 12.7.2 21G1974 x86_64
Xcode 14.2 14C18

Verification

Have you

• followed our Commit Message Guidelines?
• squashed and minimized your commits?
• checked that there aren't other open pull requests for the same change?
• referenced existing tickets on Trac with full URL?
• checked your Portfile with port lint --nitpick?
• tried existing tests with sudo port test?
• tried a full install with sudo port -vst install?
• tested basic functionality of all binary files?
• checked that the Portfile's most important variants haven't been broken?

[macportsbot]

Notifying maintainers:
@michaelld for port stellarium.

[pmetzger]

Three days have elapsed and the changes seem okay, so I'm going to merge this.