gSystem->IsFileInIncludePath() does not understand quoted paths (esp. crucial since ROOT-7226 fix auto-quotes paths)

Dear ROOTers,

I found today that:

gSystem->IsFileInIncludePath()

does not work for quoted paths. An example is a path like this:

root [0] std::cout << gInterpreter->GetIncludePath() << std::endl;

-I"/etc/root" -I"/usr/include/root" -I"/usr/lib/gcc/x86_64-pc-linux-gnu/4.9.2/include/g++-v4" -I"/usr/lib/gcc/x86_64-pc-linux-gnu/4.9.2/include/g++-v4/x86_64-pc-linux-gnu" -I"/usr/lib/gcc/x86_64-pc-linux-gnu/4.9.2/include/g++-v4/backward" -I"/etc/root/cling" -I"." -I"/var/tmp/portage/sci-physics/root-9999/work/root-9999" -I"/var/tmp/portage/sci-physics/root-9999/work/root-9999/"

The function "IsInIncludePath" does not find files in these quoted paths.

This is especially important since the fix for ROOT-7226 auto-quotes all interpreter-related paths, so they are all not treated correctly anymore.

I found that using a test-code in our framework which works using the following structure:
1. Load some libraries with ROOT / analysis framework classes inside. 2. Walk through full class-table.
3. Get TClass info for each.
4. Call GetDeclFileName() on these, this should give me the name of the header file as found in include-path during compilation.
5. Call gSystem->IsFileInIncludePath(classHeaderFileName.Data(), &absPath), this should never fail.

I can also try to write up some shorter code for that later if you are interested in having a ROOT-test of that functionality.

Cheers,
Oliver