fix GSL link problem with gr-dtv and gr-atsc

[horvathp]

Fix build issues on OS X due to missing library path for libgsl in gr-dtv and gr-atsc.

[jmcorgan]

Can you post the build/link output for when this fails?

[horvathp]

https://lists.gnu.org/archive/html/discuss-gnuradio/2016-10/msg00186.html

I'm seeing the same (not macports but homebrew, i.e., it is not macports-specific)

A similar fix has been adopted for gr-fec:
c300392

[bastibl]

AFAIS , this adds support to link with something like
-L<path> -lgsl
Wouldn't it be better to adapt FindGSL.cmake to use the full path for the library, i.e.,
-l<path>gsl.so
I think most other libraries also use the full path (QT, Python, ...).

[horvathp]

Basti, I think if gsl is a dependency then the path to the gsl library needs to be propagated down (-Lxxx as you mention) to the respective modules (in this case specifically to the swig part). This is exactly what this patch does. The fact that these gnuradio modules compile on most Linux platforms is just a fortunate accident, it owes to the fact that on these systems libgsl.so will be found in the common lib directory (e.g., /usr/lib/x86_64-linux-gnu). That is not the case on OS X with macports or homebrew, where almost every library will be installed into a separate subdirectory.

On the other hand it is true that propagating the full path would be the desired behavior, as can be seen in the previously linked output for the boost libraries. However, baking in libXXX.so is not a good idea just because on OS X .dylib is the conventional extension (as far as I understand the OS X development model :)

The current findGSL.cmake is pretty complicated due to hacks related to non-gcc windows compilers. On OS X, exactly like under Linux, it will query
gsl-config --libs
to get the -Lxxx and -lxxx flags, which, in my case, yields
-L/usr/local/Cellar/gsl1/1.16/lib -lgsl -lgslcblas
exactly as seen on the linker command line before.

So I guess one can either stick with the current linux-style -lgsl stuff (which, as I learned, on OS X means linking with libgsl.dylib instead of libgsl.a) and ensure -Lxxx gets propagated down to every module that depends on gsl, or one has to add a specific clause to findGSL.cmake in order to bypass gsl-config or postprocess its output.

[jmcorgan]

I think for now we can continue with the proposed fix, and take a look at how FindGSL.cmake works later.

Also, @horvathp, when submitting bug fixes, we ask that they be branched of the maint branch where possible and submitted with a distinct branch name, that is, not 'maint'.

For this one I'm going to cherry pick the commit manually onto maint, but please keep it in mind for next time, and thanks for the fix.

[horvathp]

OK, I'll be more careful next time. Thanks, Jonathan.