How do I instruct CMake to look for libraries installed by MacPorts?

Add /opt/local/lib, and any other likely install paths, to the set of paths searched by cmake in your CMakeLists.txt file:

set(CMAKE_LIBRARY_PATH ${CMAKE_LIBRARY_PATH} /opt/local/lib)

This appends /opt/local/lib to the set of paths in which cmake searches for libraries. This CMAKE_LIBRARY_PATH technique will affect all find_library commands after you set the variable.

For a more surgical, library-by-library approach, modify the individual find_library commands:

find_library(Foo foo
    PATHS /opt/local/lib)

Note that this does not hardcode /opt/local/lib as the only place to look for the library. Rather, it merely appends /opt/local/lib to the set of locations in which to search for the library. I often end up adding many such paths, covering the locations observed on all of the machines I know about. See the find_library documentation for more variations on this theme.

You might also wish to change CMAKE_INCLUDE_PATH, which affects the behavior of find_file() and find_path() commands.

Per @Nerdling's "Do NOT hardcode" comment on the accepted solution, here's a proposal to detect the MacPorts prefix path.

MyModule.cmake

# Detect if the "port" command is valid on this system; if so, return full path
EXECUTE_PROCESS(COMMAND which port RESULT_VARIABLE DETECT_MACPORTS OUTPUT_VARIABLE MACPORTS_PREFIX ERROR_QUIET OUTPUT_STRIP_TRAILING_WHITESPACE)

IF (${DETECT_MACPORTS} EQUAL 0)
    # "/opt/local/bin/port" doesn't have libs, so we get the parent directory
    GET_FILENAME_COMPONENT(MACPORTS_PREFIX ${MACPORTS_PREFIX} DIRECTORY)

    # "/opt/local/bin" doesn't have libs, so we get the parent directory
    GET_FILENAME_COMPONENT(MACPORTS_PREFIX ${MACPORTS_PREFIX} DIRECTORY)

    # "/opt/local" is where MacPorts lives, add `/lib` suffix and link
    LINK_DIRECTORIES(${LINK DIRECTORIES} ${MACPORTS_PREFIX}/lib)

    MESSAGE("WINNING!: ${MACPORTS_PREFIX}/lib")
ENDIF()

# Recommendation, also add a "brew --prefix" custom command to detect a homebrew build environment

I added a toolchain file for "Darwin" which adds the necessary include and library paths. I was hoping for something a little more automatic but at least it solves the problem.

darwin.cmake:

SET(CMAKE_SYSTEM_NAME Darwin)

# Add MacPorts
INCLUDE_DIRECTORIES(/opt/local/include)
LINK_DIRECTORIES(/opt/local/lib)

CMake needs to respect the DYLD_LIBRARY_PATH environment variable, which is the equivalent of the LD_LIBRARY_PATH environment variable on Linux. Your DYLD_LIBRARY_PATH needs to have the proper path to find libraries installed by MacPorts.