Use "modern" CMake facilities to manage includes and libs

Use "modern" CMake facilities to manage includes and libs

* Use target_link_libraries to specify dependencies (include paths,
  compiler options, etc)

* Export targets and provide `ginac-config.cmake` so the following
  `CMakeLists.txt` is enough to build a program using GiNaC:

    cmake_minimum_required(VERSION 3.1)
    project(foo)
    find_package(ginac REQUIRED)
    add_executable(foo foo.cpp)
    target_link_libraries(foo PRIVATE ginac::ginac)

* GiNaC can be included as a (CMake) subproject. One can put GiNaC
  sources (either from git or a tarball) into a subdirectory (say,
  `ginac`) and use the following CMakeLists.txt to build everything:

    cmake_minimum_required(VERSION 3.1)
    project(foo)
    add_subdirectory(ginac)
    add_executable(foo foo.cpp)
    target_link_libraries(foo PRIVATE ginac::ginac)