[PATCH][RFC] Extend locations where to seach for Fortran pre-include.

[Joseph Myers]

On Tue, 20 Nov 2018, Jakub Jelinek wrote:

> hardcoding /usr/include looks just very wrong here.  That should always be
> dependent on the configured prefix or better be relative from the driver,
> gcc should be relocatable.  Or at least come from configure.  It should e.g.
> honor the sysroot stuff etc.
> 
> That said, I think you need somebody familiar with the driver, perhaps
> Joseph?

I'd sort of expect structures like those in cppdefault.[ch] to describe 
the relevant Fortran directories and their properties (such as being 
sysrooted or not - and if sysrooted, I suppose you'll want to make sure 
SYSROOT_HEADERS_SUFFIX_SPEC is properly applied).

If this preinclude doesn't pass through the C preprocessor, directories in 
which it is searched for will need multilib or multiarch suffixes.  
(Multilib suffixes on include directories for C are more or less an 
implementation detail of how fixed headers are arranged in the case where 
sysroot headers suffixes are used; they aren't really expected to be a 
stable interface such that third-party software might install anything 
using them, but I'm not sure if this preinclude is meant to come from 
external software or be installed by GCC.  Multiarch suffixes, for systems 
using Debian/Ubuntu-style multiarch directory arrangements, *are* intended 
as a stable interface.  And multilib *OS* suffixes 
(-print-multi-os-directory) are a stable interface, but only really 
suitable for libraries, not headers, because they are paths relative to 
lib/ such as ../lib64.)

-- 
Joseph S. Myers
joseph@codesourcery.com