Custom library install path for MacOS and CMake

CustomInstallPath.cmake

# Suppose under certain circumstances you want to set the library path explicitly to something, but
# in the usual case, you would like it to remain as an rpath.

# This adds a variable to the install script that can be overridden 
install(CODE "
 if(NOT DEFINED CUSTOM_LIBRARY_PREFIX)
   set(CUSTOM_LIBRARY_PREFIX \"@rpath\")
 endif()
")
set_property(TARGET my_target PROPERTY INSTALL_NAME_DIR "\${CUSTOM_LIBRARY_PREFIX}")

# Now when install is run, it keeps the same behaviour as before (setting the path to @rpath/my_target.dylb)
# But suppose you manually invoke the install script like:

cmake -DCUSTOM_LIBRARY_PREFIX=/my/custom/path -P cmake_install.cmake

# Then now the library path will be /my/custom/path/my_target.dylib instead of @rpath/my_target.dylib