cmake: Introduce interface libraries to encapsulate common flags

Also add a sanity check for non-encapsulated (directory-wide) build
properties.

CMakeLists.txt

@@ -59,8 +59,21 @@ set(CMAKE_CXX_STANDARD 20)
 set(CMAKE_CXX_STANDARD_REQUIRED ON)
 set(CMAKE_CXX_EXTENSIONS OFF)
 
+list(APPEND CMAKE_MODULE_PATH ${PROJECT_SOURCE_DIR}/cmake/module)
+
 set(configure_warnings)
 
+# The core_interface library aims to encapsulate common build flags.
+# It is a usage requirement for all targets except for secp256k1, which
+# gets its flags by other means.
+add_library(core_interface INTERFACE)
+add_library(core_interface_relwithdebinfo INTERFACE)
+add_library(core_interface_debug INTERFACE)
+target_link_libraries(core_interface INTERFACE
+  $<$<CONFIG:RelWithDebInfo>:core_interface_relwithdebinfo>
+  $<$<CONFIG:Debug>:core_interface_debug>
+)
+
 message("\n")
 message("Configure summary")
 message("=================")
@@ -73,3 +86,6 @@ if(configure_warnings)
     endforeach()
     message("  ******\n")
 endif()
+
+# We want all build properties to be encapsulated properly.
+include(WarnAboutGlobalProperties)