Fix XL compilers on non-AIX machines.

Linking broken on non-AIX machines when using XL compilers due to those
machines not using the CreateExportList tool.  Made use of this tool
conditional on finding it.

Modules/Compiler/XL.cmake

@@ -35,14 +35,19 @@ macro(__compiler_xl lang)
   set(CMAKE_${lang}_CREATE_PREPROCESSED_SOURCE "<CMAKE_${lang}_COMPILER> <DEFINES> <FLAGS> -E <SOURCE> > <PREPROCESSED_SOURCE>")
   set(CMAKE_${lang}_CREATE_ASSEMBLY_SOURCE     "<CMAKE_${lang}_COMPILER> <DEFINES> <FLAGS> -S <SOURCE> -o <ASSEMBLY_SOURCE>")
 
-  # The compiler front-end passes all object files, archive files, and shared
-  # library files named on the command line to CreateExportList to create a
-  # list of all symbols to be exported from the shared library.  This causes
-  # all archive members to be copied into the shared library whether they are
-  # needed or not.  Instead we run the tool ourselves to pass only the object
-  # files so that we export only the symbols actually provided by the sources.
-  set(CMAKE_${lang}_CREATE_SHARED_LIBRARY
-    "${CMAKE_XL_CreateExportList} <OBJECT_DIR>/objects.exp <OBJECTS>"
-    "<CMAKE_${lang}_COMPILER> <CMAKE_SHARED_LIBRARY_${lang}_FLAGS> <LANGUAGE_COMPILE_FLAGS> <LINK_FLAGS> <CMAKE_SHARED_LIBRARY_CREATE_${lang}_FLAGS> -Wl,-bE:<OBJECT_DIR>/objects.exp <CMAKE_SHARED_LIBRARY_SONAME_${lang}_FLAG><TARGET_SONAME> -o <TARGET> <OBJECTS> <LINK_LIBRARIES>"
-    )
+  # CMAKE_XL_CreateExportList is part of the AIX XL compilers but not the linux ones.
+  # If we found the tool, we'll use it to create exports, otherwise stick with the regular
+  # create shared library compile line.
+  if (CMAKE_XL_CreateExportList)
+    # The compiler front-end passes all object files, archive files, and shared
+    # library files named on the command line to CreateExportList to create a
+    # list of all symbols to be exported from the shared library.  This causes
+    # all archive members to be copied into the shared library whether they are
+    # needed or not.  Instead we run the tool ourselves to pass only the object
+    # files so that we export only the symbols actually provided by the sources.
+    set(CMAKE_${lang}_CREATE_SHARED_LIBRARY
+      "${CMAKE_XL_CreateExportList} <OBJECT_DIR>/objects.exp <OBJECTS>"
+      "<CMAKE_${lang}_COMPILER> <CMAKE_SHARED_LIBRARY_${lang}_FLAGS> <LANGUAGE_COMPILE_FLAGS> <LINK_FLAGS> <CMAKE_SHARED_LIBRARY_CREATE_${lang}_FLAGS> -Wl,-bE:<OBJECT_DIR>/objects.exp <CMAKE_SHARED_LIBRARY_SONAME_${lang}_FLAG><TARGET_SONAME> -o <TARGET> <OBJECTS> <LINK_LIBRARIES>"
+      )
+  endif()
 endmacro()