In simple terms, add_subdirectory() is replaced with ExternalProject_Add().
This change is inspired by https://crascit.com/2015/07/25/cmake-gtest/
with one difference, no download, using the source we already have.
Before this change, make debug-test must be preceded by make clean.
Otherwise, a subsequent build would be polluted by cmake options made
by tests/gtest/.
Also removed the changed compiler flags. My test build did not have
the affected warnings.
Keep the immediate direct deps at the library that depends on them,
declare deps as PUBLIC so that targets that link against that library
get the library's deps as transitive deps.
Break dep cycle between blockchain_db <-> crytonote_core.
No code refactoring, just hide cycle from cmake so that
it doesn't complain (cycles are allowed only between
static libs, not shared libs).
This is in preparation for supproting BUILD_SHARED_LIBS cmake
built-in option for building internal libs as shared.
CMake config file written, but was unable to test/get it working
properly because of a bug in CMake with functions related to
find_package. Simple "-lunbound" flag used in its stead for now. May
not build on non-Linux systems, not sure yet.