cmake: auto-detect MacPorts install layout for OpenMP, Qt6, and rpath

MacPorts nests libomp's headers/library under an extra 'libomp'
subdirectory and installs the Qt6 CMake package configs under
libexec/qt6 rather than lib/cmake, so find_package(OpenMP)/find_package(Qt6...)
did not see them without manually injecting OpenMP_ROOT/CMAKE_PREFIX_PATH
on the command line. Detect the MacPorts prefix (/opt/local) automatically,
same as already done for Homebrew, including in the install RPATH fallback.

CMakeLists.txt

@@ -165,6 +165,17 @@ find_package(LibXml2 REQUIRED)
 
 #--- check for OpenMP ---------------------------------------------------------
 if (try_OpenMP)
+  #--- AppleClang has no built-in OpenMP runtime; MacPorts' 'libomp' port
+  #--- nests its headers/library one level deeper (include/libomp, lib/libomp)
+  #--- than the <prefix>/include, <prefix>/lib layout FindOpenMP.cmake looks
+  #--- for by default, so point it there directly if the port is installed. -
+  #--- (Only takes effect if the user hasn't already set OpenMP_ROOT.) ------
+  if (APPLE AND NOT DEFINED OpenMP_ROOT AND EXISTS "/opt/local/lib/libomp")
+    set(OpenMP_ROOT "/opt/local/lib/libomp" CACHE PATH "OpenMP root directory")
+  endif ()
+  if (APPLE AND EXISTS "/opt/local/include/libomp")
+    list(APPEND CMAKE_PREFIX_PATH "/opt/local/include/libomp")
+  endif ()
   find_package(OpenMP)
   if (OpenMP_CXX_FOUND)
     add_compile_definitions(HAVE_GOMP)
@@ -210,6 +221,12 @@ endif (DKS_FOUND)
 
 #--- check for Qt -------------------------------------------------------------
 if (qt_based_tools)
+  #--- MacPorts installs the actual Qt6 tree, including its CMake package
+  #--- config files, under libexec/qt6 rather than directly under lib/cmake,
+  #--- so find_package(Qt6...) below would not see it otherwise. ------------
+  if (APPLE AND EXISTS "/opt/local/libexec/qt6/lib/cmake")
+    list(APPEND CMAKE_PREFIX_PATH "/opt/local/libexec/qt6/lib/cmake")
+  endif ()
   # check for any Qt, i.e. AUTO
   if (qt_version STREQUAL AUTO)
     # try Qt6
@@ -383,7 +400,8 @@ if (UNIX AND NOT APPLE)
   set(CMAKE_INSTALL_RPATH "${rpath}")
 elseif (APPLE)
   #--- on macOS the 3rd party libs typically come from Homebrew, which lives in
-  #--- /opt/homebrew (Apple silicon) or /usr/local (Intel) -------------------
+  #--- /opt/homebrew (Apple silicon) or /usr/local (Intel), or from MacPorts,
+  #--- which lives in /opt/local on both architectures -----------------------
   set(rpath ${CMAKE_INSTALL_RPATH})
   if (EXISTS "/opt/homebrew/lib")
     string(APPEND rpath ";/opt/homebrew/lib")
@@ -391,6 +409,9 @@ elseif (APPLE)
   if (EXISTS "/usr/local/lib")
     string(APPEND rpath ";/usr/local/lib")
   endif ()
+  if (EXISTS "/opt/local/lib")
+    string(APPEND rpath ";/opt/local/lib")
+  endif ()
   set(CMAKE_INSTALL_RPATH "${rpath}")
 endif ()
 

src/classes/CMakeLists.txt

@@ -6,8 +6,9 @@ set(MUSRFIT_INC ${CMAKE_SOURCE_DIR}/src/include)
 # Hence, target_include_directories cannot be used here because, targets are 
 # setup only afterwards.
 include_directories(${MUSRFIT_INC})
-#--- note: FFTW3_INCLUDE is deliberately NOT added here. On a Homebrew based
-#--- macOS it is /opt/homebrew/include, i.e. the complete 3rd party header tree,
+#--- note: FFTW3_INCLUDE is deliberately NOT added here. On a Homebrew or
+#--- MacPorts based macOS it is /opt/homebrew/include or /opt/local/include,
+#--- i.e. the complete 3rd party header tree,
 #--- and a global include_directories() would put it in front of the headers of
 #--- every target below. The dictionaries below get it via their OPTIONS, and
 #--- everything else via the FFTW3::FFTW3 imported target. ---------------------