* add: CMake configuration files
* doc: Variables affecting CMake configuration installation
* fix: Rename CMake module and project -> subproject macro
The problem is that macro/function definitions in CMake have global scope. Therefore, use a custom macro name rather than replacing the standard CMake command.
* enh: Set LibLBFGS_LIBRARIES variable in LibLBFGSConfig.cmake
For (older) libLBFGS installations that do not contain the CMake package configuration file, a FindLibLBFGS.cmake module will be needed which then sets LibLBFGS_LIBRARIES to the path of the actual library file instead of an imported CMake build target.
* doc: Update CMakeLists.txt comment
* fix: Do not inherit CMake options from parent project
The parent project must explicitly set ${PROJECT_NAME}_${varname} before including the subproject to the value of the same named parent option ${varname} if super- and sub-project should both use the same value.
* fix: Export of targets as part of parent project
* fix: Do not import targets when liblbfgs TARGET exists
* enh: Prefix Subproject module functions, use unique target names and non-imported target ALIAS
* enh: Remove check if PROJECT_NAME is defined
* fix: Update documentation of CMakeLists.txt
* enh: Document LBFGS_INSTALL_HEADERS
