Add compile option to enable large strings support

[davidwendt]

Description

Adds CUDF_LARGE_STRINGS_ENABLED compile-time option to enable large strings support. This changes the default behavior of the LIBCUDF_LARGE_STRINGS_ENABLED environment variable -- when the variable is not set. If CUDF_LARGE_STRINGS_ENABLED is compiled on then setting LIBCUDF_LARGE_STRINGS_ENABLED=0 will turn it off at runtime. If CUDF_LARGE_STRINGS_ENABLED is not compiled on then setting LIBCUDF_LARGE_STRINGS_ENABLED=1 will turn it on at runtime.

This PR sets CUDF_LARGE_STRINGS_ENABLED=ON by default in the build.sh

Checklist

• [x] I am familiar with the Contributing Guidelines.
• [x] New or existing tests cover these changes.
• [x] The documentation is up to date with these changes.