Cmake should respect DLIB_INSTALL_DIR but it doesn't

[jhiemstrawisc]

When trying to install LotMan with

mkdir build && cd build
cmake -DLIB_INSTALL_DIR=/usr/lib64 ..
make -j`nproc` install

the library should be installed to /usr/lib64. Instead, a warning is printed that LIB_INSTALL_DIR is not used by the project, and the library ends up in /usr/local/lib64.

A workaround for this is instead to install like so:

    mkdir build && cd build 
    cmake -DCMAKE_INSTALL_PREFIX=/usr .. 
    make -j`nproc` install

However, we should probably sort this out so that LIB_INSTALL_DIR is respected.