differentiate local includes and system includes

[firewave]

Currently DUI only contains includePaths which are used for any include no matter if local or system.

There's also no support for -isystem in the simplecpp command-line tool.

[firewave]

Oh - it looks like the existing code is actually correct.

https://gcc.gnu.org/onlinedocs/cpp/Invocation.html#Invocation

Directories specified with -iquote apply only to the quote form of the directive, #include "file". Directories specified with -I, -isystem, or -idirafter apply to lookup for both the #include "file" and #include <file> directives.