Added the possibility to compile with a version of GCC older than 8.0

[farox1]

Fixes # . Added the possibility to compile with a version of GCC older than 8.0 where filesystem is found under
<experimental/filesystem>. Tested on my Linux box (CPU AMD E1) with Mint 19.3 and GCC 7.5

Changes proposed in this pull request:

• Makefile Added USE_OLDGCC option
• Main.cpp and amiberry_filesys.cpp fixed define for OLDGCC and also fixed some lines where filesystem where used.

@midwan

[farox1]

Hi i have redo the pull-request. hope this is fine now.

Side note:
I have other changes in the work, like adding Pyra support.
Pyra is a Handheld with OMAP5 SOC (ARM Cortex A15)...it use the same CPU options than Odroid xu4 and have a screen of 5 inchs with a resolution of 1280*720....but the standard gui is really tiny at this resolution on this screen ...so i changed the gui to adapt to this resolution. it work good but i need to do some more testing before...
Are you interested in this changes ? If not maybe i could add a pull-request only to add Pyra options.
If you want to see how it look at this resolution .... follow this link on the DragonBox Pyra forum
Let me know.
Thanks

[midwan]

@farox1 Thanks for this, looks good!
Regarding support for the Pyra, no problem to add it - as long as it doesn't mess anything up for the other platforms. :)
The 800x600 resolution for the GUI was chosen because it's a good fit for the 4:3 screens that the native Amiga screenmodes use. If someone uses a real CRT monitor for example, it will show correctly there (vs a widescreen resolution, which will not).

If we add support for a larger GUI, it would have to be optional.