r.in.gdal/r.external: add basic support for GDALs GDT_Int8

[ninsbl]

This PR adds basic support for importing GDT_Int8 datatype via GDAL.

I am no C-programmer, so I would be happy if a knowledgeable person could look into this. I am especially unsure about the #ifndef part (note that 14 is the constant representing GDT_Int8).

Addresses #4230

[neteler]

It compiles fine on Fedora 39 with GDAL 3.7.3.

CI: There is an issue with this approach on systems with older GDAL:

gdal.c: In function ‘Rast_get_gdal_link’:
gdal.c:132:5: error: case value ‘14’ not in enumerated type ‘GDALDataType’ [-Werror=switch]
  132 |     case GDT_Int8:
      |     ^~~~
cc1: all warnings being treated as errors

I am afraid that only a GDAL version check might work, like:

#if GDAL_VERSION_NUM >= 3000000

or by combining in a test (see raster/r.out.gdal/main.c for variants):

#if GDAL_VERSION_MAJOR >= 3

and

#if GDAL_VERSION_MINOR >= 7
?

[rouault]

#if GDAL_VERSION_NUM >= 3000000

Better use
#if GDAL_VERSION_NUM >= GDAL_COMPUTE_VERSION(3,7,0)

[ninsbl]

Thanks @neteler and @rouault ! Your feedback is very much appreciated. I tried to implement yor suggestions in the latest commits...

[nilason]

Stefan, you should also add #include <stdint.h> (which is where the C99 fixed width integer types are defined, see more) to get_row.c.

[ninsbl]

Stefan, you should also add #include <stdint.h> (which is where the C99 fixed width integer types are defined, see more) to get_row.c.

Thanks for the hint. Added in 1565b1d

I also added a test for this case.

[ninsbl]

Thanks for your approval @nilason . I had to add a switch to skip the test for systems with GDAL < 3.7 (like Ubuntu 22.04, where tests failed). That commit dismissed your review, it would be great if you could re-approve...

[nilason]

Should this be backported?