cmake_command: Add command to EVAL a CMake script as a string

Help/command/cmake_command.rst

@@ -9,6 +9,7 @@ Synopsis
 .. parsed-literal::
 
   cmake_command(`INVOKE`_ <command> [<args>...])
+  cmake_command(`EVAL`_ CODE <code>...)
 
 Introduction
 ^^^^^^^^^^^^
@@ -16,8 +17,10 @@ Introduction
 This command will call meta-operations on built-in CMake commands or
 those created via the :command:`macro` or :command:`function` commands.
 
-Invoking
-^^^^^^^^
+``cmake_command`` does not introduce a new variable or policy scope.
+
+Invoking Commands
+^^^^^^^^^^^^^^^^^
 
 .. _INVOKE:
 
@@ -38,3 +41,50 @@ is equivalent to
 .. code-block:: cmake
 
   message(STATUS "Hello World!")
+
+Evaluating Code
+^^^^^^^^^^^^^^^
+
+.. _EVAL:
+
+.. code-block:: cmake
+
+  cmake_command(EVAL CODE <code>...)
+
+Evaluates the ``<code>...`` as CMake code.
+
+For example, the code:
+
+.. code-block:: cmake
+
+  set(A TRUE)
+  set(B TRUE)
+  set(C TRUE)
+  set(condition "(A AND B) OR C")
+
+  cmake_command(EVAL CODE "
+    if (${condition})
+      message(STATUS TRUE)
+    else()
+      message(STATUS FALSE)
+    endif()"
+  )
+
+is equivalent to
+
+.. code-block:: cmake
+
+  set(A TRUE)
+  set(B TRUE)
+  set(C TRUE)
+  set(condition "(A AND B) OR C")
+
+  file(WRITE ${CMAKE_CURRENT_BINARY_DIR}/eval.cmake "
+    if (${condition})
+      message(STATUS TRUE)
+    else()
+      message(STATUS FALSE)
+    endif()"
+  )
+
+  include(${CMAKE_CURRENT_BINARY_DIR}/eval.cmake)