Safeguard CMake targets

[mmoelle1]

Dear OpenQL developers,

could you please consider safeguarding dependency targets in the form

if(NOT target)
  add_subdirectory(target)
endif()

Currently, it is not possible to have both OpenQL and QX as CMake subproject due to duplicate targets:

• func-gen (CMake Error at build/_deps/qx-simulator-src/deps/libqasm/src/cqasm/func-gen/CMakeLists.txt:4 (add_executable):)
• cqasm_objlib (CMake Error at build/_deps/qx-simulator-src/deps/libqasm/src/cqasm/CMakeLists.txt:122 (add_library):)
• cqasm (CMake Error at build/_deps/qx-simulator-src/deps/libqasm/src/cqasm/CMakeLists.txt:176 (add_library):)