Further work to upgrade the custom supplment for GMT 6.1

Changes in this PR:

3b0c848: Update the root CMakeLists.txt
7e56761: Add CMake codes for building the custom library.
112beb5: The library needs to find GMT and netCDF header files and libraries. Thus copy FindGMT.cmake and FindNETCDF.cmake from the GMT repository
f2d5712: Add the glue code gmt_custom_glue.c, generated by gmt --new-glue.
8c1c973: Fix GMT_Get_Values. GMT_Get_Value was renamed to GMT_Get_Values and has one more parameter.
3c77ac4: Remove the inclusion of custom_version.h, which was removed in PR #2
9c03cb9: Replace GMT_STR16 with GMT_VF_LEN
9fbc355: Rename nx and ny in GMT_GRD_HEADER to n_columns and n_rows

Now I can succefully build the custom library on macOS, and run the modules.

Remaning issues:

In 8c1c973, GMT_Get_Values need one more parameter than before. I'm not sure if the parameters I added are correct.
All the modules use gmt_show_name_and_purpose, which is not available for type 4 supplements

GenericMappingTools / custom-supplements