Add install.optional flag

Author: mrexodia

docs/cmake-toml.md

@@ -204,4 +204,5 @@ component = "mycomponent"
 files = ["content/my.png"]
 dirs = ["include"]
 configs = ["Release", "Debug"]
+optional = false
 ```

include/project_parser.hpp

@@ -128,6 +128,7 @@ struct Install {
     std::vector<std::string> configs;
     std::string destination;
     std::string component;
+    bool optional = false;
 };
 
 struct Subdir {

src/cmake_generator.cpp

@@ -1045,8 +1045,9 @@ void generate_cmake(const char *path, const parser::Project *parent_project) {
                 component_name = inst.targets.front();
             }
             auto component = std::make_pair("COMPONENT", component_name);
+            auto optional = inst.optional ? "OPTIONAL" : "";
             ConditionScope cs(gen, inst.condition);
-            cmd("install")(targets, dirs, files, configs, destination, component);
+            cmd("install")(targets, dirs, files, configs, destination, component, optional);
         }
     }
 

src/project_parser.cpp

@@ -619,6 +619,7 @@ Project::Project(const Project *parent, const std::string &path, bool build) {
             i.optional("configs", inst.configs);
             i.required("destination", inst.destination);
             i.optional("component", inst.component);
+            i.optional("optional", inst.optional);
             installs.push_back(inst);
         }
     }