COMP: add rpath information to MacOS compilation rules (#2948)

- Since the Apple SIP (System Integrity Protection) clears environment
  variables such as DYLD_LIBRARY_PATH env variable, a number of
  workarounds have been used to provide shadow values.

  However, for a more robust installation using the -rpath at
  compilation time appears to be the best solution.

  Add MacOS-specific rpath handling (@loader_path, @executable_path)
  to the wmake rules.

  Library paths handled:
    - FOAM_USER_LIBBIN
    - FOAM_SITE_LIBBIN
    - FOAM_FOAM_EXT_LIBBIN, FOAM_EXT_LIBBIN/FOAM_MPI
    - FOAM_LIBBIN (implicitly)

  The executable rpaths are handled assuming a structure of
     install-path/bin
     install-path/lib

- no rpath added for c-only compilations since there are currently
  no c-only libraries or executables with dynamic loading

  The current default for MacOS is the link with rpath information.
  This can be disabled by including `~rpath` in the WM_COMPILE_CONTROL

wmake/rules/darwin64Clang/c++

@@ -16,10 +16,16 @@ cctoo       = $(Ctoo)
 cpptoo      = $(Ctoo)
 cxxtoo      = $(Ctoo)
 
-LINK_LIBS   = $(c++DBUG)
 
-LINKLIBSO   = $(CC) $(c++FLAGS) -Wl,-dylib,-undefined,dynamic_lookup
+# Linking:
+# with or without rpath components on MacOS
+# - current default is with rpath unless explicitly disabled
 
-LINKEXE     = $(CC) $(c++FLAGS) -Wl,-execute,-undefined,dynamic_lookup
+ifneq (,$(findstring ~rpath,$(WM_COMPILE_CONTROL)))
+    include $(DEFAULT_RULES)/link-c++
+
+else
+    include $(DEFAULT_RULES)/link-rpath-c++
+endif
 
 #------------------------------------------------------------------------------

wmake/rules/darwin64Clang/link-c++

+#------------------------------------------------------------------------------
+
+LINK_LIBS   = $(c++DBUG)
+
+LINKLIBSO   = $(CC) $(c++FLAGS) -Wl,-dylib,-undefined,dynamic_lookup
+
+LINKEXE     = $(CC) $(c++FLAGS) -Wl,-execute,-undefined,dynamic_lookup
+
+#------------------------------------------------------------------------------

wmake/rules/darwin64Clang/link-rpath-c++

+#------------------------------------------------------------------------------
+include $(DEFAULT_RULES)/rpath
+
+LINK_LIBS   = $(c++DBUG)
+
+LINKLIBSO   = $(CC) $(c++FLAGS) \
+    $(PROJECT_RPATH) -dynamiclib \
+    -install_name @rpath/$(notdir $(LIB)$(EXT_SO)) \
+    -Wl,-dylib,-undefined,dynamic_lookup
+
+LINKEXE     = $(CC) $(c++FLAGS) \
+    $(subst @loader_path,@executable_path/../lib,$(PROJECT_RPATH)) \
+    -Wl,-execute,-undefined,dynamic_lookup
+
+#------------------------------------------------------------------------------

wmake/rules/darwin64Clang/rpath

+#------------------------------------------------------------------------------
+
+# Compile-time rpath information:
+
+PROJECT_RPATH :=
+
+ifneq (,$(strip $(FOAM_USER_LIBBIN)))
+    PROJECT_RPATH += -rpath $(FOAM_USER_LIBBIN)
+endif
+ifneq (,$(strip $(FOAM_SITE_LIBBIN)))
+    PROJECT_RPATH += -rpath $(FOAM_SITE_LIBBIN)
+endif
+ifneq (,$(strip $(FOAM_EXT_LIBBIN)))
+    PROJECT_RPATH += -rpath $(FOAM_EXT_LIBBIN)
+    ifneq (,$(strip $(FOAM_MPI)))
+        PROJECT_RPATH += -rpath $(FOAM_EXT_LIBBIN)/$(FOAM_MPI)
+    endif
+endif
+
+# MacOS relative rpath:
+# Encode as @loader_path and recompose as @executable_path as needed
+
+PROJECT_RPATH += -rpath @loader_path
+ifneq (,$(strip $(FOAM_MPI)))
+    PROJECT_RPATH += -rpath @loader_path/$(FOAM_MPI)
+endif
+# Fallback for stubs (eg, missing MPI)
+PROJECT_RPATH += -rpath @loader_path/dummy
+
+#------------------------------------------------------------------------------