add_compile_definitions: add new command

This command manages preprocessor definitions at directory level and
supports generator expressions.

Fixes: #15374

Source/cmAddCompileDefinitionsCommand.h

@@ -0,0 +1,31 @@
+/* Distributed under the OSI-approved BSD 3-Clause License.  See accompanying
+   file Copyright.txt or https://cmake.org/licensing for details.  */
+#ifndef cmAddCompileDefinitionsCommand_h
+#define cmAddCompileDefinitionsCommand_h
+
+#include "cmConfigure.h" // IWYU pragma: keep
+
+#include <string>
+#include <vector>
+
+#include "cmCommand.h"
+
+class cmExecutionStatus;
+
+class cmAddCompileDefinitionsCommand : public cmCommand
+{
+public:
+  /**
+   * This is a virtual constructor for the command.
+   */
+  cmCommand* Clone() override { return new cmAddCompileDefinitionsCommand; }
+
+  /**
+   * This is called when the command is first encountered in
+   * the CMakeLists.txt file.
+   */
+  bool InitialPass(std::vector<std::string> const& args,
+                   cmExecutionStatus& status) override;
+};
+
+#endif