AIX: Add platform id variables 'AIX' and 'CMAKE_HOST_AIX'

This allows projects to use `if(AIX)` blocks instead of spelling
out `if(CMAKE_SYSTEM_NAME STREQUAL "AIX")`.  We already have similar
variables for many other platforms.

Help/manual/cmake-variables.7.rst

@@ -323,6 +323,7 @@ Variables that Describe the System
 .. toctree::
    :maxdepth: 1
 
+   /variable/AIX
    /variable/ANDROID
    /variable/APPLE
    /variable/BORLAND
@@ -331,6 +332,7 @@ Variables that Describe the System
    /variable/CMAKE_CL_64
    /variable/CMAKE_COMPILER_2005
    /variable/CMAKE_HOST_APPLE
+   /variable/CMAKE_HOST_AIX
    /variable/CMAKE_HOST_BSD
    /variable/CMAKE_HOST_EXECUTABLE_SUFFIX
    /variable/CMAKE_HOST_LINUX

Help/release/4.0.rst

@@ -41,6 +41,9 @@ Commands
 Variables
 ---------
 
+* The :variable:`AIX` and :variable:`CMAKE_HOST_AIX` variables are
+  now set to true when the target or host system is AIX, respectively.
+
 * Linker flag variables learned to support the ``LINKER:`` prefix:
 
   * :variable:`CMAKE_EXE_LINKER_FLAGS`

Help/variable/AIX.rst

@@ -0,0 +1,6 @@
+AIX
+---
+
+.. versionadded:: 4.0
+
+Set to true when the target system is AIX.

Help/variable/CMAKE_HOST_AIX.rst

@@ -0,0 +1,6 @@
+CMAKE_HOST_AIX
+--------------
+
+.. versionadded:: 4.0
+
+Set to true when the host system is AIX.

Modules/CMakeSystemSpecificInitialize.cmake

@@ -17,6 +17,7 @@ unset(MSYS)
 unset(WIN32)
 unset(BSD)
 unset(LINUX)
+unset(AIX)
 
 # The CMAKE_EFFECTIVE_SYSTEM_NAME is used to load compiler and compiler
 # wrapper configuration files. By default it equals to CMAKE_SYSTEM_NAME

Modules/Platform/AIX-Initialize.cmake

@@ -1 +1,2 @@
+set(AIX 1)
 set(UNIX 1)

Source/cmStateSnapshot.cxx

@@ -325,6 +325,11 @@ void cmStateSnapshot::SetDefaultDefinitions()
   this->SetDefinition("CMAKE_HOST_LINUX", "1");
 #endif
 
+#if defined(_AIX)
+  this->SetDefinition("AIX", "1");
+  this->SetDefinition("CMAKE_HOST_AIX", "1");
+#endif
+
   this->SetDefinition("CMAKE_MAJOR_VERSION",
                       std::to_string(cmVersion::GetMajorVersion()));
   this->SetDefinition("CMAKE_MINOR_VERSION",