added export of package so that source builds can be found directly

[mirkow]

I added the cmake export of the package so that source builds can be found directly without the new to specify OsqpEigen_DIR or install the package.

[traversaro]

By default export(PACKAGE) writes file outside of the build directory during build time, and this is quite a counter-intuitive and error-prone behavior. Since CMake 3.14, the command only works if the CMAKE_EXPORT_PACKAGE_REGISTRY variable is explicitly set (see https://cmake.org/cmake/help/latest/policy/CMP0090.html and https://gitlab.kitware.com/cmake/cmake/merge_requests/3041). I think that if you really want these feature, we should at least only call export(PACKAGE) if CMAKE_EXPORT_PACKAGE_REGISTRY is set, as done for example in https://github.com/robotology/ycm/pull/269 .

[S-Dafarra]

I would also change the base branch to devel and eventually bump the patch version.

[traversaro]

@mirkow are you still interested in replying/following up this PR, or can we close it? Thanks!

[mirkow]

I don't need this anymore, thanks