[CMake] Add path to Fortran libraries to RPATH for h2root on macOS

So far, we have only solved this problem by adding the path to
`DYLD_LIBRARY_PATH` when running the test, but this is not a complete
solution, as the libraries won't be found automatically if this is not
set. That affects in particular the users that want to use `h2root` from
the install tree.

Using the RPATH mechanism is more appropriate here.

Author: guitargeek

hist/hbook/CMakeLists.txt

@@ -31,3 +31,17 @@ ROOT_STANDARD_LIBRARY_PACKAGE(Hbook
     TreePlayer
     RIO
 )
+
+# This is needed in particular for macOS, where the path of the GNU Fortran
+# library might not be in the default DYLD_LIBRARY_PATH, but it gets picked
+# up with enable_language(Fortran).
+if(APPLE AND CMAKE_Fortran_COMPILER_ID STREQUAL "GNU")
+  # Compute path to quadmath
+  execute_process(
+    COMMAND ${CMAKE_Fortran_COMPILER} -print-file-name=libgfortran.dylib
+    OUTPUT_VARIABLE LIBGFORTRAN_PATH
+    OUTPUT_STRIP_TRAILING_WHITESPACE)
+  get_filename_component(LIBGFORTRAN_DIR "${LIBGFORTRAN_PATH}" DIRECTORY)
+  set_property(TARGET Hbook APPEND PROPERTY BUILD_RPATH "${LIBGFORTRAN_DIR}")
+  set_property(TARGET Hbook APPEND PROPERTY INSTALL_RPATH "${LIBGFORTRAN_DIR}")
+endif()

main/CMakeLists.txt

@@ -55,6 +55,20 @@ if(fortran AND CMAKE_Fortran_COMPILER)
   ROOT_EXECUTABLE(g2root g2root.f LIBRARIES minicern CMAKENOEXPORT)
   set_target_properties(g2root PROPERTIES COMPILE_FLAGS "-w")
   ROOT_EXECUTABLE(h2root h2root.cxx LIBRARIES Core RIO Net Hist Graf Graf3d Gpad Tree Matrix MathCore Thread minicern CMAKENOEXPORT)
+
+  # This is needed in particular for macOS, where the path of the GNU Fortran
+  # library might not be in the default DYLD_LIBRARY_PATH, but it gets picked
+  # up with enable_language(Fortran).
+  if(APPLE AND CMAKE_Fortran_COMPILER_ID STREQUAL "GNU")
+    # Compute path to quadmath
+    execute_process(
+      COMMAND ${CMAKE_Fortran_COMPILER} -print-file-name=libgfortran.dylib
+      OUTPUT_VARIABLE LIBGFORTRAN_PATH
+      OUTPUT_STRIP_TRAILING_WHITESPACE)
+    get_filename_component(LIBGFORTRAN_DIR "${LIBGFORTRAN_PATH}" DIRECTORY)
+    set_property(TARGET h2root APPEND PROPERTY BUILD_RPATH "${LIBGFORTRAN_DIR}")
+    set_property(TARGET h2root APPEND PROPERTY INSTALL_RPATH "${LIBGFORTRAN_DIR}")
+  endif()
 endif()
 
 file(GLOB utils RELATIVE ${CMAKE_CURRENT_SOURCE_DIR} python/root*)