[CMAKE] Add compiler flags to some build configuration only
Summary:
This adds new facilities to add flags or definitions that only apply to
some build configuration. This is especially useful to pass debug only
flags.
Test Plan:
cmake -GNinja .. -DCMAKE_BUILD_TYPE=Debug ninja -v check
Ensure the flags and definitions are correctly passed on the compilation
lines.
Reviewers: #bitcoin_abc, deadalnix
Reviewed By: #bitcoin_abc, deadalnix
Differential Revision: https://reviews.bitcoinabc.org/D4949