cmake: fix shared lib build with NAG Fortran on macOS

On Apple, the shared library link rule for Fortran used GCC-specific flags:
  -x f95-cpp-input   (GCC preprocessing flag; NAG does not support it)
  -Wl,-shared        (Linux linker flag; macOS needs -dynamiclib)

The fix is NAG-specific — other non-GNU compilers (Intel, flang, Cray,
etc.) are left alone since they may handle the existing code paths fine.

Fix in two places (both already inside if(APPLE) / if(APPLE AND BUILD_STATIC_LIBS)):

1. Before add_library() (affects the main CMake Fortran shared lib rule):
   For NAG Fortran, substitute CMAKE_C_CREATE_SHARED_LIBRARY so the
   C compiler link rule is used instead.

2. Inside the APPLE AND BUILD_STATIC_LIBS block (the static->shared
   conversion trick for long argument lists): add an elseif(NAG) branch
   that uses the C compiler with -dynamiclib and auto-detects the NAG
   Fortran runtime (libf72rts) from the compiler's directory so that
   NAGf90_* symbols resolve.

Tested with NAG 7.2.7243 on macOS/arm64.

Author: mathomp4

CMakeLists.txt

@@ -263,6 +263,14 @@ if(APPLE)
   if(NOT NOFORTRAN)
     set(CMAKE_Fortran_USE_RESPONSE_FILE_FOR_OBJECTS ON)
     set(CMAKE_Fortran_USE_RESPONSE_FILE_FOR_LIBRARIES ON)
+    # NAG Fortran does not support the GCC-specific -x f95-cpp-input flag
+    # used in the shared library link rule below.  Override
+    # CMAKE_Fortran_CREATE_SHARED_LIBRARY early (before targets are defined)
+    # to use the C compiler link rule instead; the shared library contains
+    # only C and assembler objects so this is safe.
+    if(CMAKE_Fortran_COMPILER_ID MATCHES "NAG")
+      set(CMAKE_Fortran_CREATE_SHARED_LIBRARY ${CMAKE_C_CREATE_SHARED_LIBRARY})
+    endif()
   endif()
   set(CMAKE_NINJA_FORCE_RESPONSE_FILE ON)
 endif()
@@ -360,9 +368,32 @@ if(APPLE AND BUILD_STATIC_LIBS)
   if(NOT NOFORTRAN)
     set(CMAKE_Fortran_CREATE_STATIC_LIBRARY ${CREATE_STATIC_LIBRARY_COMMAND})
     if(BUILD_SHARED_LIBS)
-      set(CMAKE_Fortran_CREATE_SHARED_LIBRARY
-	"sh -c 'echo \"\" | ${CMAKE_Fortran_COMPILER} -o dummy.o -c -x f95-cpp-input - '"
-	"sh -c '${CMAKE_Fortran_COMPILER} -fpic -shared -Wl,-all_load -Wl,-force_load,${STATIC_PATH} dummy.o -o ${SHARED_PATH} ${OMP_LIB}'")
+      if(CMAKE_Fortran_COMPILER_ID MATCHES "GNU")
+        set(CMAKE_Fortran_CREATE_SHARED_LIBRARY
+          "sh -c 'echo \"\" | ${CMAKE_Fortran_COMPILER} -o dummy.o -c -x f95-cpp-input - '"
+          "sh -c '${CMAKE_Fortran_COMPILER} -fpic -shared -Wl,-all_load -Wl,-force_load,${STATIC_PATH} dummy.o -o ${SHARED_PATH} ${OMP_LIB}'")
+      elseif(CMAKE_Fortran_COMPILER_ID MATCHES "NAG")
+        # NAG does not support -x f95-cpp-input, and macOS needs -dynamiclib
+        # rather than -Wl,-shared.  Use the C compiler as the linker driver
+        # and add the NAG Fortran runtime so NAGf90_* symbols resolve.
+        # Locate the NAG Fortran runtime (libf<XY>rts) without hardcoding the
+        # version digits: NAG 7.1 ships libf71rts, 7.2 ships libf72rts, etc.
+        get_filename_component(_NAG_FC_DIR "${CMAKE_Fortran_COMPILER}" DIRECTORY)
+        set(_NAG_RTS_DIR "${_NAG_FC_DIR}/../lib/NAG_Fortran")
+        file(GLOB _NAG_RTS_CANDIDATES
+             "${_NAG_RTS_DIR}/libf*rts.dylib"
+             "${_NAG_RTS_DIR}/libf*rts.a")
+        if(_NAG_RTS_CANDIDATES)
+          list(GET _NAG_RTS_CANDIDATES 0 _NAG_RTS_LIB)
+          get_filename_component(_NAG_RTS_NAME "${_NAG_RTS_LIB}" NAME_WE)
+          string(REGEX REPLACE "^lib" "" _NAG_RTS_LIBNAME "${_NAG_RTS_NAME}")
+          set(_NAG_RTS_FLAGS "-L${_NAG_RTS_DIR} -l${_NAG_RTS_LIBNAME}")
+        else()
+          set(_NAG_RTS_FLAGS "")
+        endif()
+        set(CMAKE_Fortran_CREATE_SHARED_LIBRARY
+          "sh -c '${CMAKE_C_COMPILER} -dynamiclib -Wl,-all_load -Wl,-force_load,${STATIC_PATH} -o ${SHARED_PATH} ${OMP_LIB} ${_NAG_RTS_FLAGS}'")
+      endif()
     endif()
   else()
     set(CMAKE_C_CREATE_STATIC_LIBRARY ${CREATE_STATIC_LIBRARY_COMMAND})