Improve ability to build custom supplements about custom-supplements

[PaulWessel]

Particularly interested in @seisman's thought on this to minimize maintenance.

[seisman]

When I tried to build the custom shared library today (mkdir build; cd build; cmake ..) I got errors about not finding stdio.h, so something very fundamental is missing.

This is what I just got:

-- Performing Test memchrExists
-- Performing Test memchrExists - Failed
-- ANSI C header files - not found
-- Using CFLAGS = ' -O3 -DNDEBUG'
CMake Warning (dev) at CMakeLists.txt:65 (get_property):
  Policy CMP0026 is not set: Disallow use of the LOCATION target property.
  Run "cmake --help-policy CMP0026" for policy details.  Use the cmake_policy
  command to set the policy and suppress this warning.

  The LOCATION property should not be read from target "customlib".  Use the
  target name directly with add_custom_command, or use the generator
  expression $<TARGET_FILE>, as appropriate.

This warning is for project developers.  Use -Wno-dev to suppress it.

*  Options:
*  GMT library             : /Users/seisman/local/GMT/lib/libgmt.dylib
*  GMT include dir         : /Users/seisman/local/GMT/include/gmt
*
*  Locations:
*  Custom name             : custom
*  CUSTOM library          : custom.so
*  Installing CUSTOM in    : /usr/local
*  CUSTOM_SHARE_PATH       : /usr/local/
-- Configuring done
-- Generating done
-- Build files have been written to: /Users/seisman/Gits/gmt/custom-supplements/build

$ make
Scanning dependencies of target customlib
[ 16%] Building C object src/CMakeFiles/customlib.dir/gmt_custom_module.c.o
/Users/seisman/Gits/gmt/custom-supplements/src/gmt_custom_module.c:51:41: error: expected expression
        {"gmtaverage", "gmtaverage", "custom", , "Block average (x,y,z) data tables by mean, median, or mode estimation"},
                                               ^
/Users/seisman/Gits/gmt/custom-supplements/src/gmt_custom_module.c:52:41: error: expected expression
        {"gmtmercmap", "gmtmercmap", "custom", , },
                                               ^
/Users/seisman/Gits/gmt/custom-supplements/src/gmt_custom_module.c:53:39: error: expected expression
        {"gmtparser", "gmtparser", "custom", , },
                                             ^
/Users/seisman/Gits/gmt/custom-supplements/src/gmt_custom_module.c:54:41: error: expected expression
        {"grdfourier", "grdfourier", "custom", , },
                                               ^
4 errors generated.
make[2]: *** [src/CMakeFiles/customlib.dir/gmt_custom_module.c.o] Error 1
make[1]: *** [src/CMakeFiles/customlib.dir/all] Error 2
make: *** [all] Error 2

[PaulWessel]

Thanks I will fix that in a bit - side-tracked by Chair business...

[seisman]

Separately from that repo, I think it would be great if we could extend the support we have in the main GMT CMake setup where we have EXTRA_BUILD_DIRS and EXTRA_MODULES_SUPPL. What I have in mind is the ability for developers to add a separate supplement dir to this mix, but have it build as a separate *.so file and not become part of supplements.so. This would simplify the whole build for such developers since they can simply rely on the main GMT cmake setup. How could we do this? Allow a symbolic link called custom to be placed in src (and ignored by .gitignore) and let our Cmake determine if there is such a thing then build custom.so?

I think it's a good idea for developers who want to write GMT plugins for their own use. We can add more cmake variables, e.g. BUILD_CUSTOM_LIBS, CUSTOM_LIB_NAME, CUSTOM_DIRS, to configure how custom plugins are found.

[PaulWessel]

I am thinking that some of our supplements (e.g., spotter) may be too specialized and only has one maintainer (me) and that it will not last very long into the future. Maybe that one and maybe all of them should be separate shared libs and supplements.so goes away,

[seisman]

I'm OK with that, as long as it's transparent to end-users.

[leouieda]

I have a feeling that whatever is built on this repo will always be out of date with the main repo. GMT development moves really fast and breaks things like this quite often. It might be worth moving things to the main repo to try to avoid that. Or at least have CI build nightly on this to send notifications when things break against master.

As part of making GMT more sustainable, I really like the idea of making supplements truly supplemental. Having a GMT federation instead of putting the burden on the current core is a good idea. And it encourages new people to take ownership. But that requires making people feel like this is their project too, not just ours (which is kind of the feeling I get from the forum comments).