Always install external headers.

[matz-e]

Prepares for NEURON + NMODL, where we will need to provide at the minimum Eigen.

Fixes #2970.

[sonarcloud[bot]]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[azure-pipelines[bot]]

✔️ 5fa294f805bdc655fe9c98e3631d02d17b277a56 -> Azure artifacts URL

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 67.25%. Comparing base (ce50582) to head (5fa294f).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #2972 +/- ## ========================================== - Coverage 67.26% 67.25% -0.01% ========================================== Files 572 572 Lines 104895 104896 +1 ========================================== - Hits 70555 70553 -2 - Misses 34340 34343 +3 ```

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[bbpbuildbot]

Logfiles from GitLab pipeline #221075 (:white_check_mark:) have been uploaded here!

Status and direct links:

• :white_check_mark: mac_m1_cmake_build: [cmake, ON, OFF, OFF, address]
• :white_check_mark: spack_setup
• :white_check_mark: build:nmodl
• :white_check_mark: build:neuron:nmodl:intel:legacy
• :white_check_mark: build:neuron:nmodl:intel:shared
• :white_check_mark: build:neuron:nmodl:nvhpc:acc:legacy
• :white_check_mark: build:neuron:nmodl:nvhpc:acc:shared
• :white_check_mark: build:neuron:nmodl:nvhpc:omp:legacy
• :white_check_mark: build:neuron:nmodl:nvhpc:omp
• :white_check_mark: test:neuron:nmodl:intel:legacy
• :white_check_mark: test:neuron:nmodl:intel:shared
• :white_check_mark: test:neuron:nmodl:nvhpc:acc:legacy
• :white_check_mark: test:neuron:nmodl:nvhpc:acc:shared
• :white_check_mark: test:neuron:nmodl:nvhpc:omp:legacy
• :white_check_mark: test:neuron:nmodl:nvhpc:omp