* Fix CMake package config file
There are currently a few issues with how the COIN-OR dependencies are
resolved when using `find_package(ortools CONFIG)` to add or-tools to
an external CMake project.
* With CMake >=3.9.6, we try to find the Clp and Cbc packages using the
CONFIG mode of `find_package`. This always fails since neither
library provides a CMake config file.
* If we address the above issue by instead using the MODULE mode of
`find_package`, then we need to add `FindCbc.cmake` and
`FindCpl.cmake` scripts to the CMAKE_MODULE_PATH in order to teach
CMake how to find these dependencies. I propose that or-tools should
install these scripts alongside its config file so that they're
available to external CMake projects.
* Finally, the `FindCbc.cmake` script included with or-tools defines the
variable `CBC_FOUND` when it's successful, rather than `Cbc_FOUND` as
expected (CMake variables are case-sensitive). The `FindCpl.cmake`
script has a similar issue. As a result, even when the above two
points are addressed, `find_package(ortools CONFIG)` will still fail
because CMake erroneously thinks that these two dependencies weren't
succesfully found.
This commit attempts to address the above issues with the following
changes:
* The or-tools CMake config file is modified to allow searching for Cbc
and Cpl using the MODULE mode of `find_package`.
* `FindCbc.cmake` and `FindCpl.cmake` are installed with the CMake
package config files and are added to the CMAKE_MODULE_PATH if
`USE_COINOR` was truthy.
* The `FindCbc.cmake` and `FindCpl.cmake` files are modified to change
the case of variables they export.
* Only install FindXXX.cmake modules if COIN-OR support was enabled
