Help: Convert some genex names to explicit cross-references

Since commit c2dc7e0f53 (Help: Convert genex documentation to sphinx
domain objects, 2021-01-15) we can use `:genex:` cross-references to
link to named generator expressions.  Update some places to do this.

This is meant to demonstrate the capability, not as a comprehensive
sweep.

Help/command/add_custom_command.rst

@@ -111,8 +111,8 @@ The options are:
 
   Arguments to ``COMMAND`` may use
   :manual:`generator expressions <cmake-generator-expressions(7)>`.
-  Use the ``TARGET_FILE`` generator expression to refer to the location of
-  a target later in the command line (i.e. as a command argument rather
+  Use the :genex:`TARGET_FILE` generator expression to refer to the location
+  of a target later in the command line (i.e. as a command argument rather
   than as the command to execute).
 
   Whenever one of the following target based generator expressions are used as

Help/command/add_custom_target.rst

@@ -84,8 +84,8 @@ The options are:
 
   Arguments to ``COMMAND`` may use
   :manual:`generator expressions <cmake-generator-expressions(7)>`.
-  Use the ``TARGET_FILE`` generator expression to refer to the location of
-  a target later in the command line (i.e. as a command argument rather
+  Use the :genex:`TARGET_FILE` generator expression to refer to the location
+  of a target later in the command line (i.e. as a command argument rather
   than as the command to execute).
 
   Whenever one of the following target based generator expressions are used as

Help/command/target_precompile_headers.rst

@@ -34,7 +34,7 @@ Repeated calls for the same ``<target>`` will append items in the order called.
 
 Projects should generally avoid using ``PUBLIC`` or ``INTERFACE`` for targets
 that will be :ref:`exported <install(EXPORT)>`, or they should at least use
-the ``$<BUILD_INTERFACE:...>`` generator expression to prevent precompile
+the :genex:`$<BUILD_INTERFACE:...>` generator expression to prevent precompile
 headers from appearing in an installed exported target.  Consumers of a target
 should typically be in control of what precompile headers they use, not have
 precompile headers forced on them by the targets being consumed (since
@@ -74,7 +74,7 @@ Arguments to ``target_precompile_headers()`` may use "generator expressions"
 with the syntax ``$<...>``.
 See the :manual:`cmake-generator-expressions(7)` manual for available
 expressions.
-The ``$<COMPILE_LANGUAGE:...>`` generator expression is particularly
+The :genex:`$<COMPILE_LANGUAGE:...>` generator expression is particularly
 useful for specifying a language-specific header to precompile for
 only one language (e.g. ``CXX`` and not ``C``).  In this case, header
 file names that are not explicitly in double quotes or angle brackets

Help/policy/CMP0026.rst

@@ -9,12 +9,12 @@ determine the eventual location of build targets.  This relies on the
 assumption that all necessary information is available at
 configure-time to determine the final location and filename of the
 target.  However, this property is not fully determined until later at
-generate-time.  At generate time, the ``$<TARGET_FILE>`` generator
+generate-time.  At generate time, the :genex:`$<TARGET_FILE>` generator
 expression can be used to determine the eventual :prop_tgt:`LOCATION` of a target
 output.
 
 Code which reads the :prop_tgt:`LOCATION` target property can be ported to
-use the ``$<TARGET_FILE>`` generator expression together with the
+use the :genex:`$<TARGET_FILE>` generator expression together with the
 :command:`file(GENERATE)` subcommand to generate a file containing
 the target location.
 

Help/policy/CMP0051.rst

@@ -3,7 +3,7 @@ CMP0051
 
 .. versionadded:: 3.1
 
-List TARGET_OBJECTS in SOURCES target property.
+List :genex:`TARGET_OBJECTS` in SOURCES target property.
 
 CMake 3.0 and lower did not include the ``TARGET_OBJECTS``
 :manual:`generator expression <cmake-generator-expressions(7)>` when

Help/prop_tgt/INSTALL_NAME_DIR.rst

@@ -12,5 +12,5 @@ This property is initialized by the value of the variable
 created.
 
 This property supports :manual:`generator expressions <cmake-generator-expressions(7)>`.
-In particular, the ``$<INSTALL_PREFIX>`` generator expression can be used to set the
-directory relative to the install-time prefix.
+In particular, the :genex:`$<INSTALL_PREFIX>` generator expression can be
+used to set the directory relative to the install-time prefix.