Please add cmake variables allowing the user to specify if to require an external package or not

[yurivict]

For each external dependency please add a tri-state variable USE_{depencency-name}={ON/OFF/AUTO}.

In the context of the package build it should be possible to require a dependency to be present or not to be used, for example:

• USE_CMINPACK=ON would require cminpack and would fail if it isn't present
• USE_CMINPACK=OFF would ignore cminpack even if it is present
• USE_CMINPACK=AUTO would use cminpack when it is present (current behavior)

Please see an example here: https://cmake.org/pipermail/cmake/2016-October/064342.html

[jschueller]

you can use CMAKE_REQUIRE_FIND_PACKAGE_CMinpack=ON if you want it to fail when not present (and USE_CMINPACK=ON/OFF for the other two cases)