Properly check for builtin

Summary: Previously, the function to check for symbols was used, but this is wrong as builtins are not symbols.

Test Plan:

cmake -GNinja ..

Check that the configure file now has the builtins set to 1.

Reviewers: schancel, #bitcoin_abc

Reviewed By: schancel, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D843