The use of `PACKAGE_NAME` for the project's variable name is
problematic, as this name is commonly used in CMake's interface
variables. If third-party CMake code handles with scopes improperly,
our `PACKAGE_NAME` variable could end up with an unexpected value.
This change avoids such conflicts by renaming all `PACKAGE_*` variables
to `CLIENT_*`.
This option replaces --with-boost-process
This prepares external signer support to be disabled by default.
It adds a configure option to enable this feature and to check
if Boost::Process is present.
This also exposes ENABLE_EXTERNAL_SIGNER to the test suite via test/config.ini