Fix the way target.headers is merged into target.sources

- It would crash if you did not specify target.sources
- Headers with condition would be merged incorrectly

include/project_parser.hpp

@@ -67,7 +67,6 @@ struct Target {
     TargetType type = target_last;
     std::string type_name;
 
-    ConditionVector headers;
     ConditionVector sources;
 
     // https://cmake.org/cmake/help/latest/manual/cmake-commands.7.html#project-commands

src/project_parser.cpp

@@ -487,9 +487,18 @@ Project::Project(const Project *parent, const std::string &path, bool build) {
             throw std::runtime_error(format_key_error(error, target.type_name, t.find("type")));
         }
 
-        t.optional("headers", target.headers);
         t.optional("sources", target.sources);
 
+        // Merge the headers into the sources
+        ConditionVector headers;
+        t.optional("headers", headers);
+        for (const auto &itr : headers) {
+            auto &dest = target.sources[itr.first];
+            for (const auto &jtr : itr.second) {
+                dest.push_back(jtr);
+            }
+        }
+
         t.optional("compile-definitions", target.compile_definitions);
         t.optional("private-compile-definitions", target.private_compile_definitions);
 
@@ -514,12 +523,6 @@ Project::Project(const Project *parent, const std::string &path, bool build) {
         t.optional("precompile-headers", target.precompile_headers);
         t.optional("private-precompile-headers", target.private_precompile_headers);
 
-        if (!target.headers.empty()) {
-            auto &sources = target.sources.nth(0).value();
-            const auto &headers = target.headers.nth(0)->second;
-            sources.insert(sources.end(), headers.begin(), headers.end());
-        }
-
         t.optional("condition", target.condition);
         t.optional("alias", target.alias);