ENH: Add support to find Headers in Apple Frameworks

The header locations in apple frameworks require manipulation
of the directory name in a '<pkg/myhdr.h>' include defintion.

The header file is located at <pkg.framework/Headers/myhdr.h>
in a framework.

Author: hjmjohnson

simplecpp.cpp

@@ -3006,6 +3006,34 @@ static std::string openHeader(std::ifstream &f, const simplecpp::DUI &dui, const
         if (!path.empty())
             return path;
     }
+#ifdef __APPLE__
+    // a named lambda function to insert the ".framework/Headers" part for apple frameworks
+    auto get_apple_framework_relative_path= [](std::string appleFrameworkHeader) -> std::string
+    {
+        // try the Framework path on apple OS, if there is a path in front
+        const size_t slashPos = appleFrameworkHeader.find('/');
+        if (slashPos != std::string::npos)
+        {
+            constexpr auto framework_separator{ ".framework/Headers" };
+            appleFrameworkHeader.insert(slashPos, framework_separator);
+        }
+        return appleFrameworkHeader;
+    };
+
+    // on Apple, try to find the header in the framework path
+    // Convert <includePath>/PKGNAME/myHeader -> <includePath>/PKGNAME.framework/Headers/myHeader
+    const std::string appleFrameworkHeader = get_apple_framework_relative_path(header);
+    if (appleFrameworkHeader != header)
+    {
+        for (const auto & includePath: dui.includePaths)
+        {
+            const std::string frameworkCandidatePath = includePath + '/' + appleFrameworkHeader;
+            std::string simplePath = openHeaderDirect(f,  simplecpp::simplifyPath(frameworkCandidatePath ));
+            if (!simplePath.empty())
+                return simplePath;
+        }
+    }
+#endif // __APPLE__
     return "";
 }