Adapt Cling C++ Header Search Paths to Flatpak

This adds additional directories to the CMake variable
CLING_CXX_HEADERS, whose content is later on compiled into
the ROOT libraries.

Cling requires access to the header files even at runtime,
but a Flatpak runtime environment does not contain them,
only the SDK used at compilation time does.

The /usr tree inside a Flatpak sanbox is immutable, everything
the application needs in addition to the underlying runtime has
to be shipped in /app.

This patch adds these paths under /app to the search paths of Cling
so that it does work inside a Flatpak sandbox.

This behaviour is controlled by the new boolean build option
“BUILD_FOR_FLATPAK”.

However, this patch alone does not do the trick. The Flatpak build manifest
needs to contain commands to create the “/app/include” directory and copy
all the include files there from the SDK as well as to install the include
files from ROOT under “/app/include/root”.