GNU: Use -fvisibility on AIX when supported

Revise the logic from commit v3.7.0-rc1~173^2~2 (GNU: Do not use
-fvisibility on AIX or HP-UX, 2016-09-03) to add a version check.
The GCC 7 release notes [1] state that visibility support has been
added for AIX 7.1 and above.

[1] https://gcc.gnu.org/gcc-7/changes.html

Modules/Platform/AIX-GNU.cmake

@@ -24,5 +24,7 @@ macro(__aix_compiler_gnu lang)
   set(CMAKE_${lang}_USE_IMPLICIT_LINK_DIRECTORIES_IN_RUNTIME_PATH 1)
 
   set(CMAKE_${lang}_LINK_FLAGS "-Wl,-bnoipath")
-  unset(CMAKE_${lang}_COMPILE_OPTIONS_VISIBILITY)
+  if(CMAKE_${lang}_COMPILER_VERSION VERSION_LESS 7 OR CMAKE_SYSTEM_VERSION VERSION_LESS 7.1)
+    unset(CMAKE_${lang}_COMPILE_OPTIONS_VISIBILITY)
+  endif()
 endmacro()

Tests/RunCMake/GenerateExportHeader/GEH.cmake

@@ -53,7 +53,16 @@ endif()
 
 add_subdirectory(lib_shared_and_static)
 
-add_compiler_export_flags()
+if(CMAKE_SYSTEM_NAME MATCHES "AIX" AND CMAKE_CXX_COMPILER_ID STREQUAL "GNU"
+   AND CMAKE_CXX_COMPILE_OPTIONS_VISIBILITY)
+  # With GNU 7 on AIX, passing -fvisibility=hidden when driving the
+  # linker for a shared library drops the so init/destruct symbols.
+  # Just use the modern approach instead of testing the macro.
+  set(CMAKE_CXX_VISIBILITY_PRESET hidden)
+  set(CMAKE_VISIBILITY_INLINES_HIDDEN 1)
+else()
+  add_compiler_export_flags()
+endif()
 
 set(CMAKE_RUNTIME_OUTPUT_DIRECTORY ${CMAKE_CURRENT_BINARY_DIR})
 set(CMAKE_RUNTIME_OUTPUT_DIRECTORY_DEBUG ${CMAKE_CURRENT_BINARY_DIR})