Remove "SYSTEM" from the include_directories() CMakeLists.txt entries.

[madscientist42]

When you use SYSTEM in an include_directories directive in a CMake file, this causes CMake to generate an -isystem flag entry for that given directory. Per FSF, this changes the search order of things on the system search pathing so that #include_next <> does not work correctly in most contexts, including a cross-compile one.

The FSF explicitly states that -isystem should not be used for this purpose except very special cases (Typically the ones for GCC itself...) and you should use -I instead as it arrives at the desired goals in general without breaking the #include_next search behavior. Removing "SYSTEM" from the directives lets CMake use -I instead.

[Code7R]

Oh yes, please!!

I stumbled upon this stupid -isystem flag generation hack in some other projects already and it's a real mess. It might not be visible with an older compiler immediately but starting with gcc 7 or so it starts wreaking havoc unless patched by compiler integrator somewhere.