[Patch, Build+Fortran] PR54725 - correctly set TARGET_SYSTEM_ROOT for CPP

[Tobias Burnus]

gfortran was ignoring the TARGET_SYSTEM_ROOT and thus searched in 
/usr/include for files specified with "#include"/"include" files and for 
.mod files.

The solution is to do in gcc/fortran/cpp.c the same as it is done in 
gcc/c-family/c-opts.c.


However, the TARGET_SYSTEM_ROOT also has to be available. For C/C++ 
that's done via gcc/Makefile.in:
   CFLAGS-c-family/c-opts.o += @TARGET_SYSTEM_ROOT_DEFINE@

For Fortran, we have to to it likewise, but slightly different:

The gcc/Makefile.in gets updated by configure and written to 
$BUILD/gcc/Makefile. At that point the @TARGET...@ has been replaced by 
the actual value.

For Fortran, the gcc/fortran/Make-lang.in is included in 
$BUILD/gcc/Makefile:
   include $(LANG_MAKEFRAGS)

Thus, we cannot use @TARGET_...@ as it won't get replaced. Hence, I 
create a Makefile variable in gcc/Makefile.in and set it there, which 
then in turn gets used in gcc/fortran/Make-lang.in.

Build and regtested on x86-64-gnu-linux.
OK for the trunk?

Tobias

PS: I haven't yet cross compiled and thus checked whether it indeed works.
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