[CMAKE] Move FDELT_TYPE declaration to config

Summary:
This is only used when the Glibc backward compatibility feature is
enabled, but moving the definition to the bitcoin-config.h instead of
the command line has no impact on code since the glibc_compat.cpp file
(the only callsite) is built conditionally.

This fixes a weird bug in Gitian builds, where passing the
-DFDELT_TYPE="long int" definition on the command line causes GCC to
fail. While being apparently correctly surrounded by the double quotes,
the compiler sees the int as a source file and aborts. This only
occurs when the script is read from the YAML descriptor during a Gitian
build, and cannot be reproduced by building from a shell.

Test Plan:

cmake -GNinja .. -DENABLE_GLIBC_BACK_COMPAT=ON
ninja

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D4347