Always install external headers.

[matz-e]

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

Fixes #2970.

[sonarqubecloud]

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]

✔️ 5fa294f -> Azure artifacts URL

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

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

Additional details and impacted files

@@            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:

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

[1uc]

Thank you, this seems to fix the issue.