Add $<LINK_LANGUAGE> generator expression

They can't be used when evaluating link libraries, but they can be
used for include directories and compile definitions. Later they can
be used for compile options.

Tests/CompileDefinitions/compiletest.cpp

@@ -56,6 +56,21 @@ enum {
 #error Expect PREFIX_DEF2
 #endif
 
+#ifndef LINK_CXX_DEFINE
+#error Expected LINK_CXX_DEFINE
+#endif
+#ifndef LINK_LANGUAGE_IS_CXX
+#error Expected LINK_LANGUAGE_IS_CXX
+#endif
+
+#ifdef LINK_C_DEFINE
+#error Unexpected LINK_C_DEFINE
+#endif
+#ifdef LINK_LANGUAGE_IS_C
+#error Unexpected LINK_LANGUAGE_IS_C
+#endif
+
+
 // TEST_GENERATOR_EXPRESSIONS
 #endif