Handle circular includes

Fixes #150.

Author: foonathan

src/libclang/preprocessor.cpp

@@ -1089,6 +1089,11 @@ detail::preprocessor_output detail::preprocess(const libclang_compile_config& co
         else
             xpath += *cpath;
 
+    // We strip all conditional defines and pragmas from the input, which includes the include
+    // guard. If the source includes a file, which includes itself again (for some reason), this
+    // leads to a duplicate include, as we no longer have an include guard. So we manually add one.
+    result.source += "#pragma once\n\n";
+
     position p(ts::ref(result.source), preprocessed.file.c_str());
     ts::flag in_string(false), in_char(false), first_line(true);
     while (p)