Make building executables optional with CMake.

[flederwiesel]

If you don't want to build the executables, set BUILD_SHAPELIB_EXECUTABLES to OFF when configuring with CMake.

[rouault]

In other projects of the ecosystem (namely GDAL and PROJ), we've standardized on "BUILD_APPS" for that option

BUILD_SHAPELIB_CONTRIB default value shoud likely be the same as BUILD_APPS (that is if you disable BUILD_APPS, you want contrib utilities to be disabled by default too)

[flederwiesel]

Thanks. I did not know about BUILD_APPS, as I am using the shapelib only as dependency of another project (which is not even mine :D).
As with the value of BUILD_APPS being the default for BUILD_SHAPELIB_CONTRIB, I added a comment about changing the value of the former requiring the latter to be changed explicitly, as to some this might not immediately occur, if not familiar with CMake.