hardcoded path to 'lib' in src/CMakeLists.txt

[ksebaz]

complexes++/src/CMakeLists.txt line 38 contains a hardcoded 'lib'-subpath which on some platforms might need to be, e.g., lib64. I guess CMAKE_INSTALL_LIBDIR could be used here since GNUInstallDirs is already included.

[ksebaz]

see #58