Fix eigen import in cmake

[luminoctum]

Currently, Eigen is downloaded and extracted via FetchContent. It seems that Eigen is not properly configured. This is the output message:

-- **** -- -- Configured Eigen 3.4.0 -- -- Available targets (use: make TARGET): -- ---------+-------------------------------------------------------------- -- Target | Description -- ---------+-------------------------------------------------------------- -- install | Install Eigen. Headers will be installed to: -- | / -- | Using the following values: -- | CMAKE_INSTALL_PREFIX: /usr/local -- | INCLUDE_INSTALL_DIR: include/eigen3 -- | Change the install location of Eigen headers using: -- | cmake . -DCMAKE_INSTALL_PREFIX=yourprefix -- | Or: -- | cmake . -DINCLUDE_INSTALL_DIR=yourdir -- doc | Generate the API documentation, requires Doxygen & LaTeX -- check | Build and run the unit-tests. Read this page: -- | http://eigen.tuxfamily.org/index.php?title=Tests -- blas | Build BLAS library (not the same thing as Eigen) -- uninstall| Remove files installed by the install target -- ---------+--------------------------------------------------------------

Is there any way to suppress this message during cmake?

[chengcli]

closed by #82