[Backend] Establish standalone build infrastructure

Add the thin monorepo CMake entry point and an independently configurable backend project. Enforce the Clang/C++23 toolchain, expose Make/Ninja-neutral tests and developer checks, export an installable RCCBackend package, and introduce the first Support error abstraction.

1 files changed, 40 insertions, 0 deletions

diff --git a/cmake/modules/RCCProjects.cmake b/cmake/modules/RCCProjects.cmake
new file mode 100644
index 0000000..59c4fc5
--- /dev/null
+++ b/cmake/modules/RCCProjects.cmake
@@ -0,0 +1,40 @@
+# SPDX-License-Identifier: GPL-3.0-or-later
+
+set(RCC_KNOWN_PROJECTS backend)
+set(RCC_DEFAULT_PROJECTS backend)
+
+set(
+  RCC_ENABLE_PROJECTS
+  "${RCC_DEFAULT_PROJECTS}"
+  CACHE STRING
+  "Semicolon-separated RCC projects to build, or 'all'"
+)
+
+function(rcc_add_enabled_projects RCCSourceRoot)
+  if(RCC_ENABLE_PROJECTS STREQUAL "all")
+    set(EnabledProjects ${RCC_KNOWN_PROJECTS})
+  else()
+    set(EnabledProjects ${RCC_ENABLE_PROJECTS})
+  endif()
+
+  if(NOT EnabledProjects)
+    message(FATAL_ERROR "RCC_ENABLE_PROJECTS must name at least one project")
+  endif()
+
+  list(REMOVE_DUPLICATES EnabledProjects)
+
+  foreach(Project IN LISTS EnabledProjects)
+    if(NOT Project IN_LIST RCC_KNOWN_PROJECTS)
+      message(
+        FATAL_ERROR
+          "Unknown RCC project '${Project}'. Known projects: ${RCC_KNOWN_PROJECTS}"
+      )
+    endif()
+  endforeach()
+
+  # Component order is controlled here rather than by the user-provided list.
+  # Future inter-project dependency closure will also be resolved here.
+  if("backend" IN_LIST EnabledProjects)
+    add_subdirectory("${RCCSourceRoot}/backend" "${CMAKE_BINARY_DIR}/backend")
+  endif()
+endfunction()