AIX: Properly detect host system version

CMakeDetermineSystem.cmake uses "uname -r" to get
CMAKE_HOST_SYSTEM_VERSION on AIX. AIX uses "uname -v" for the
major version and "uname -r" for the minor version.

Thanks to Chris Taylor for the suggested patch.

Closes #10749

Modules/CMakeDetermineSystem.cmake

@@ -35,7 +35,15 @@
 if(CMAKE_HOST_UNIX)
   find_program(CMAKE_UNAME uname /bin /usr/bin /usr/local/bin )
   if(CMAKE_UNAME)
-    exec_program(${CMAKE_UNAME} ARGS -r OUTPUT_VARIABLE CMAKE_HOST_SYSTEM_VERSION)
+    if(CMAKE_HOST_SYSTEM_NAME STREQUAL "AIX")
+      exec_program(${CMAKE_UNAME} ARGS -v OUTPUT_VARIABLE _CMAKE_HOST_SYSTEM_MAJOR_VERSION)
+      exec_program(${CMAKE_UNAME} ARGS -r OUTPUT_VARIABLE _CMAKE_HOST_SYSTEM_MINOR_VERSION)
+      set(CMAKE_HOST_SYSTEM_VERSION "${_CMAKE_HOST_SYSTEM_MAJOR_VERSION}.${_CMAKE_HOST_SYSTEM_MINOR_VERSION}")
+      unset(_CMAKE_HOST_SYSTEM_MAJOR_VERSION)
+      unset(_CMAKE_HOST_SYSTEM_MINOR_VERSION)
+    else()
+      exec_program(${CMAKE_UNAME} ARGS -r OUTPUT_VARIABLE CMAKE_HOST_SYSTEM_VERSION)
+    endif()
     if(CMAKE_HOST_SYSTEM_NAME MATCHES "Linux|CYGWIN.*|Darwin|^GNU$")
       exec_program(${CMAKE_UNAME} ARGS -m OUTPUT_VARIABLE CMAKE_HOST_SYSTEM_PROCESSOR
         RETURN_VALUE val)