Fix GCC path detection for Ubuntu 20.

The path scheme changed in recent Ubuntu versions.

Previously, C++ includes would be found under `/usr/include/c++/9.0`,
but now they are found in `/usr/include/c++/9`.

Change our path detection logic to look for this.

Fixes https://github.com/mono/CppSharp/issues/1392.

src/Parser/ParserOptions.cs

@@ -237,7 +237,8 @@ namespace CppSharp.Parser
             AddSystemIncludeDirs(BuiltinsDir);
             AddArguments($"-fgnuc-version={longVersion}");
 
-            string[] versions = { longVersion, shortVersion };
+            string majorVersion = shortVersion.Split('.')[0];
+            string[] versions = { longVersion, shortVersion, majorVersion };
             string[] triples = { "x86_64-linux-gnu", "x86_64-pc-linux-gnu" };
             if (compiler == "gcc")
             {