[gdal-dev] Add capability to safely link against an external libtiff (3.X) and a GDAL build with internal libtiff (4.0) support

[Even Rouault]

Folks,

Hopefully this is the solution to a long-standing problem. Below the inline 
text of http://trac.osgeo.org/gdal/ticket/4144

"""
Currently it is not safe for an application to link libgdal built with 
internal libtiff support with system libtiff (3.X), because of the difference of 
ABI. The same issue could potentially happen with libgeotiff (in 1.9.0dev 
trunk, it's at v1.3.0, whereas in the wild you mostly find it at v1.2.0).

The solution implemented consists in :
    * compiling a libtiff.so/libgeotiff.so,
    * extracting the symbols from the .so with objdump,
    * and creating a header file that does #define original_symbol_name 
gdal_original_symbol_name for each symbol. 

This process is automated by a dump_symbols.sh scripts in frmts/gtiff/libtiff 
and frmts/gtiff/libgeotiff that must be run whenever internal libtiff/libgeotiff 
is updated (and that the version includes new symbols). The generated files are 
placed in svn to avoid running the script at each build, mainly because the 
script might only work on Linux. Wouldn't work as it on MacOS. But the end-
result should be portable.

Of course, one also must make sure this header file is included at the 
appropriate places. This mainly consists in including it in the tif_config.h 
and geo_config.h, but also in the INGR, NITF, JPEG drivers happen to use libtiff 
symbols, and MrSID for libgeotiff.

The renaming of the symbols is enabled if --with-rename-internal-libtiff-
symbols is defined. (--with-rename-internal-libgeotiff-symbols for geotiff).

This can be combined with the --with-hide-internal-symbols of course so that 
those gdal_ prefixed symbols are not exported, but this is an orthogonal issue.

"""

This solution is now implemented in trunk. Testing from others appreciated ! 
Although I've tested various combinations internal/external libtiff/libgeotiff, 
with/without the new flags, there might still be issues.

Best regards,

Even