Clean up system-or-submodule handling

Fixes a bug on older cmake linking against oxenmq (older cmake hates the
direct oxenmq::oxenmq -> PkgConfig::OXENMQ alias), and also makes it
easier to handle things like nlohmann::json (which we can use from the
system *or* submodule).

Borrowed from oxen-ss/oxen-core.

CMakeLists.txt

@@ -175,39 +175,6 @@ if(NOT TARGET sodium)
   export(TARGETS sodium NAMESPACE sodium:: FILE sodium-exports.cmake)
 endif()
 
-option(FORCE_OXENC_SUBMODULE "force using oxen-encoding submodule" OFF)
-if(NOT FORCE_OXENC_SUBMODULE)
-  pkg_check_modules(OXENC liboxenc>=1.0.3 IMPORTED_TARGET)
-endif()
-
-if(OXENC_FOUND)
-  if(NOT TARGET PkgConfig::OXENC AND CMAKE_VERSION VERSION_LESS "3.21")
-    # Work around cmake bug 22180 (PkgConfig::OXENC not set if no flags needed):
-    add_library(_empty_oxenc INTERFACE)
-    add_library(oxenc::oxenc ALIAS _empty_oxenc)
-  else()
-    add_library(oxenc::oxenc ALIAS PkgConfig::OXENC)
-  endif()
-  message(STATUS "Found system liboxenc ${OXENC_VERSION}")
-else()
-  message(STATUS "using oxen-encoding submodule")
-  add_subdirectory(${CMAKE_CURRENT_SOURCE_DIR}/external/oxen-encoding)
-  add_library(oxenc::oxenc ALIAS oxenc)
-endif()
-
-option(FORCE_OXENMQ_SUBMODULE "force using oxenmq submodule" OFF)
-if(NOT FORCE_OXENMQ_SUBMODULE)
-  pkg_check_modules(OXENMQ liboxenmq>=1.2.12 IMPORTED_TARGET)
-endif()
-if(OXENMQ_FOUND)
-  add_library(oxenmq::oxenmq ALIAS PkgConfig::OXENMQ)
-  message(STATUS "Found system liboxenmq ${OXENMQ_VERSION}")
-else()
-  message(STATUS "using oxenmq submodule")
-  add_subdirectory(${CMAKE_CURRENT_SOURCE_DIR}/external/oxen-mq)
-endif()
-
-
 if(NOT APPLE)
   add_compile_options(-U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=0 -Wall -Wextra -Wno-unknown-pragmas -Wno-unused-function -Wno-deprecated-declarations -Werror=vla)
   if (CMAKE_CXX_COMPILER_ID MATCHES "Clang")

external/CMakeLists.txt

@@ -29,12 +29,38 @@ if(SUBMODULE_CHECK)
   endif()
 endif()
 
+macro(system_or_submodule BIGNAME smallname pkgconf subdir)
+  option(FORCE_${BIGNAME}_SUBMODULE "force using ${smallname} submodule" OFF)
+  if(NOT STATIC AND NOT FORCE_${BIGNAME}_SUBMODULE)
+    pkg_check_modules(${BIGNAME} ${pkgconf} IMPORTED_TARGET)
+  endif()
+  if(${BIGNAME}_FOUND)
+    add_library(${smallname} INTERFACE)
+    if(NOT TARGET PkgConfig::${BIGNAME} AND CMAKE_VERSION VERSION_LESS "3.21")
+      # Work around cmake bug 22180 (PkgConfig::THING not set if no flags needed)
+    else()
+      target_link_libraries(${smallname} INTERFACE PkgConfig::${BIGNAME})
+    endif()
+    message(STATUS "Found system ${smallname} ${${BIGNAME}_VERSION}")
+  else()
+    message(STATUS "using ${smallname} submodule")
+    add_subdirectory(${subdir})
+  endif()
+  if(NOT TARGET ${smallname}::${smallname})
+    add_library(${smallname}::${smallname} ALIAS ${smallname})
+  endif()
+endmacro()
+
+system_or_submodule(OXENC oxenc liboxenc>=1.0.3 oxen-encoding)
+system_or_submodule(OXENMQ oxenmq liboxenmq>=1.2.12 oxen-mq)
+set(JSON_BuildTests OFF CACHE INTERNAL "")
+system_or_submodule(NLOHMANN nlohmann_json nlohmann_json>=3.7.0 nlohmann)
+
+
 if(WITH_HIVE)
   add_subdirectory(pybind11 EXCLUDE_FROM_ALL)
 endif()
 
-set(JSON_BuildTests OFF CACHE INTERNAL "")
-add_subdirectory(nlohmann EXCLUDE_FROM_ALL)
 add_subdirectory(cxxopts EXCLUDE_FROM_ALL)
 add_subdirectory(date EXCLUDE_FROM_ALL)