jrl: Add cppad, cppadcg, gmp and mpfr find-modules

[ahoarau]

Taken from stack-of-tasks/pinocchio#2923

Important note:
Initially the Findcppadcg.cmake was calling find_package(cppad REQUIRED) internally.
The issue is that our jrl_export_package() cannot detect transitive find-modules, it only reacts to jrl_find_package().
As we want to keep the find modules jrl-free, they have been edited to be autonomous: Findcppadcg.cmake will look for the cppad::cppad target internally, making it NOT depend on Findcppad.cmake.

An alternative explored was on the user side, with people having to call the required finder before the other:

jrl_find_package(cppad REQUIRED)
jrl_find_package(cppadcg REQUIRED)

So that the cppad find module would get exported. This is not ideal as its difficult to enforce.

[mergify]

Tick the box to add this pull request to the merge queue (same as @mergifyio queue).

• Queue this pull request

[ahoarau]

@nim65s I need to invoke your nix magic powers on this one :-)
We need to add cppad, cppagcg, gmp and mpfr if they are available.

[nim65s]

I wonder if we should try pkg_check_modules(… IMPORTED_TARGET) (or In CMake >= 3.31, cmake_pkg_config(IMPORT …)) instead.

Some packages do not export cmake configs, but the .pc file they ship are good.

But probably we should continue using something similar to v1 find-external for now

[ahoarau]

I think the objective of the v2 was to drop pkg-config completely, but I like the idea.
Most probably in the future they will ship a cmake config, and drop pkg-config.

[nim65s]

probably not gmp or mpfr

[ahoarau]

probably not gmp or mpfr

if gmp and mpfr not possible, I need to add an aditionnal option OFF by default to avoid building this test. Let me know.

[nim65s]

I mean I don't think gmp and mpfr will ever switch to CMake