install: Add EXCLUDE_FROM_ALL option (#14921)

Let us take an example of a project that has some tests in a component
that need to be installed into a dedicated test package.  The user
expectation is that the result could be achieved by typing the
following:

    make
    make tests
    make install
    DESTDIR=/testpkgs make install-tests

However this results in test components in the default installation as
well as the testpkg.

Add an EXCLUDE_FROM_ALL option to the install() command to tell it that
the installation rule should not be included unless its component is
explicitly specified for installation.

Help/command/install.rst

@@ -45,11 +45,15 @@ signatures that specify them.  The common options are:
   is associated, such as "runtime" or "development".  During
   component-specific installation only install rules associated with
   the given component name will be executed.  During a full installation
-  all components are installed.  If ``COMPONENT`` is not provided a
-  default component "Unspecified" is created.  The default component
-  name may be controlled with the
+  all components are installed unless marked with ``EXCLUDE_FROM_ALL``.
+  If ``COMPONENT`` is not provided a default component "Unspecified" is
+  created.  The default component name may be controlled with the
   :variable:`CMAKE_INSTALL_DEFAULT_COMPONENT_NAME` variable.
 
+``EXCLUDE_FROM_ALL``
+  Specify that the file is excluded from a full installation and only
+  installed as part of a component-specific installation
+
 ``RENAME``
   Specify a name for an installed file that may be different from the
   original file.  Renaming is allowed only when a single file is
@@ -76,7 +80,8 @@ Installing Targets
            [PERMISSIONS permissions...]
            [CONFIGURATIONS [Debug|Release|...]]
            [COMPONENT <component>]
-           [OPTIONAL] [NAMELINK_ONLY|NAMELINK_SKIP]
+           [OPTIONAL] [EXCLUDE_FROM_ALL]
+           [NAMELINK_ONLY|NAMELINK_SKIP]
           ] [...])
 
 The ``TARGETS`` form specifies rules for installing targets from a
@@ -172,7 +177,7 @@ Installing Files
           [PERMISSIONS permissions...]
           [CONFIGURATIONS [Debug|Release|...]]
           [COMPONENT <component>]
-          [RENAME <name>] [OPTIONAL])
+          [RENAME <name>] [OPTIONAL] [EXCLUDE_FROM_ALL])
 
 The ``FILES`` form specifies rules for installing files for a project.
 File names given as relative paths are interpreted with respect to the
@@ -206,7 +211,8 @@ Installing Directories
           [DIRECTORY_PERMISSIONS permissions...]
           [USE_SOURCE_PERMISSIONS] [OPTIONAL] [MESSAGE_NEVER]
           [CONFIGURATIONS [Debug|Release|...]]
-          [COMPONENT <component>] [FILES_MATCHING]
+          [COMPONENT <component>] [EXCLUDE_FROM_ALL]
+          [FILES_MATCHING]
           [[PATTERN <pattern> | REGEX <regex>]
            [EXCLUDE] [PERMISSIONS permissions...]] [...])
 
@@ -282,7 +288,7 @@ Custom Installation Logic
 ::
 
   install([[SCRIPT <file>] [CODE <code>]]
-          [COMPONENT <component>] [...])
+          [COMPONENT <component>] [EXCLUDE_FROM_ALL] [...])
 
 The ``SCRIPT`` form will invoke the given CMake script files during
 installation.  If the script file name is a relative path it will be
@@ -307,7 +313,8 @@ Installing Exports
           [PERMISSIONS permissions...]
           [CONFIGURATIONS [Debug|Release|...]]
           [EXPORT_LINK_INTERFACE_LIBRARIES]
-          [COMPONENT <component>])
+          [COMPONENT <component>]
+          [EXCLUDE_FROM_ALL])
 
 The ``EXPORT`` form generates and installs a CMake file containing code to
 import targets from the installation tree into another project.