Use of embedded external dependencies

[olivier-roussel]

Following the remarks we got from the SOFA conda package PR on conda-forge (conda-forge/staged-recipes#23085), we have some problems regarding some external libraries that are embedded in SOFA code.
Here is a try to sum up all of these embedded libraries:

Lib name	Type	Location in code	License	Status
difflib	header-only	extlibs	MIT	OK
json nlohmann	header-only	extlibs	MIT (should be in a separate file)	OK. Fixed in #4272
stb	header-only	extlibs	MIT	OK
TinyXML	static	extlibs	zlib (should be in a separate file)	Externalize tinyxml dependency: #4240 (waiting for merge)
miniFlowVR	static	applications/plugins/SofaDistanceGrid/extlibs	LGPL (version ?) (should be in a separate file)	Not built by default. TODO: plugin to be externalized (#1442 )
newmat	static	applications/plugins/SofaNewmat/extlibs	type inconnu	Not built by default. TODO: plugin to be externalized (#1442 )
CImg	header-only	applications/plugins/CImgPlugin/extlibs	CeCILL-C / CeCILL	OK
csparse	static	Sofa/Component/LinearSolver/Direct/extlibs	no license file	Plugin to be externalized. WIP : #4258
metis	static	Sofa/Component/LinearSolver/Direct/extlibs	Apache v2	Externalize metis dependency : #4266 (waiting for merge)
gtest	shared	Sofa/framework/Testing/extlibs	BSD-3-Clause	Externalize ?
cxxopts	header-only	Sofa/GUI/Common (fetched from GitHub)	MIT	Add to possibility to use external dependency and prevent from fetching: Fixed in #4273
libQGLViewer	shared	Sofa/GUI/Qt	GPL	Find or use embedded code (find module being fixed in #4290)

The actions we should take to make our code compatible for the conda package on conda-forge are:

1. Fix the missing licenses files (points in bold)
2. Avoid all static libraries.

For point 2, we are not concerned by miniFlowVR and newmat, as we do not compile the corresponding plugins for the conda package, at least for now. Before considering any action on these two, do the SofaDistanceGrid and SofaNewmat plugins are still used and planned to be maintained in future releases ?

Regarding other static libs (TinyXML, csparse and metis), could we consider to switch to an external dependency instead ? For TinyXML, the current behavior is to first look for an external package, and fallback to embedded version if unavailable. This looks reasonable at first, but shouldn't we switch to fully externalized versions for maintainability ?

Last point, we were also advised to use external dependency for some header-only libraries, especially when they are already packaged in conda-forge. Could we consider also to use externalized versions for some of these (the ones easily available as external packages), such as json nlohmann and cxxopts ?

[olivier-roussel]

Edited to add libQGLViewer