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.

Source/cmInstallScriptGenerator.h

@@ -21,7 +21,7 @@ class cmInstallScriptGenerator: public cmInstallGenerator
 {
 public:
   cmInstallScriptGenerator(const char* script, bool code,
-    const char* component);
+    const char* component, bool exclude_from_all);
   virtual ~cmInstallScriptGenerator();
 
 protected: