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

- since the Apple SIP (System Integrity Protection) clears environment
  variables such as DYLD_LIBRARY_PATH, a number of workarounds have
  been used to provide shadow values. However, for a more robust
  installation using -rpath at compilation time appears to be the
  better solution.

  In addition to the usual -rpath specification with absolute file
  paths, MacOS supports (@loader_path, @executable_path) as well.
  Now default to link with rpath information for MacOS, which can be
  disabled by adding `~rpath` in WM_COMPILE_CONTROL

  Explicit library paths handled:
    - FOAM_FOAM_EXT_LIBBIN, FOAM_EXT_LIBBIN/FOAM_MPI

  The executable rpaths are handled assuming a structure of
     install-path/bin
     install-path/lib/$(FOAM_MPI)
     install-path/lib

  Absolute compile-time paths for FOAM_USER_LIBBIN, FOAM_SITE_LIBBIN
  and FOAM_LIBBIN are not handled since these are either too fragile
  (FOAM_USER_LIBBIN and FOAM_SITE_LIBBIN values) or covered via
  @loader_path anyhow (FOAM_LIBBIN).

  Since the value of FOAM_MPI is a compile-time value, this rpath
  treatment makes the installation less suitable for runtime changes
  to the MPI vendor/version.

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

wmake/rules/darwin64Clang/rpath

@@ -0,0 +1,30 @@
+#------------------------------------------------------------------------------
+
+# Compile-time rpath information:
+
+FOAM_MPI ?= dummy
+ifeq (,$(strip $(FOAM_MPI)))
+    FOAM_MPI = dummy
+endif
+
+PROJECT_RPATH :=
+
+# ThirdParty libraries (FOAM_EXT_LIBBIN) : mpi-specific and serial
+ifneq (,$(strip $(FOAM_EXT_LIBBIN)))
+    ifneq (dummy,$(strip $(FOAM_MPI)))
+        PROJECT_RPATH += -rpath $(FOAM_EXT_LIBBIN)/$(FOAM_MPI)
+    endif
+    PROJECT_RPATH += -rpath $(FOAM_EXT_LIBBIN)
+endif
+
+# project libraries (FOAM_LIBBIN) : mpi-specific and serial
+# encode as @loader_path and recompose as @executable_path as needed
+ifneq (dummy,$(strip $(FOAM_MPI)))
+    PROJECT_RPATH += -rpath @loader_path/$(FOAM_MPI)
+endif
+PROJECT_RPATH += -rpath @loader_path
+
+# Fallback for stubs libraries (largely for missing MPI) - to be found last
+PROJECT_RPATH += -rpath @loader_path/dummy
+
+#------------------------------------------------------------------------------