build: add option for external mpgen binary

This is useful for cross builds.

Note that the internal binary is still built, but it can be skipped by building
the multiprocess target directly.

Author: theuni

CMakeLists.txt

@@ -22,6 +22,8 @@ if(Libmultiprocess_ENABLE_CLANG_TIDY)
   set(CMAKE_CXX_CLANG_TIDY "${CLANG_TIDY_EXECUTABLE}")
 endif()
 
+set(EXTERNAL_MPGEN "" CACHE STRING "Use the supplied mpgen binary rather than the one built internally")
+
 include("cmake/compat_config.cmake")
 include("cmake/pthread_checks.cmake")
 include(GNUInstallDirs)

cmake/TargetCapnpSources.cmake

@@ -60,15 +60,23 @@ function(target_capnp_sources target include_prefix)
     "IMPORT_PATHS"  # multi_value_keywords
   )
 
-  if(NOT TARGET Libmultiprocess::mpgen)
-    message(FATAL_ERROR "Target 'Libmultiprocess::mpgen' does not exist.")
+  set(MPGEN_BINARY "")
+  if(EXTERNAL_MPGEN)
+    set(MPGEN_BINARY "${EXTERNAL_MPGEN}")
+    if(NOT EXISTS "${MPGEN_BINARY}")
+      message(FATAL_ERROR "EXTERNAL_MPGEN: \"${MPGEN_BINARY}\" does not exist.")
+    endif()
+  elseif(TARGET Libmultiprocess::multiprocess)
+    set(MPGEN_BINARY $<TARGET_FILE:Libmultiprocess::mpgen>)
+  else()
+    message(FATAL_ERROR "No usable mpgen. Set EXTERNAL_MPGEN or enable the internal target.")
   endif()
 
   set(generated_headers "")
   foreach(capnp_file IN LISTS TCS_UNPARSED_ARGUMENTS)
     add_custom_command(
       OUTPUT ${capnp_file}.c++ ${capnp_file}.h ${capnp_file}.proxy-client.c++ ${capnp_file}.proxy-types.h ${capnp_file}.proxy-server.c++ ${capnp_file}.proxy-types.c++ ${capnp_file}.proxy.h
-      COMMAND Libmultiprocess::mpgen ${CMAKE_CURRENT_SOURCE_DIR} ${include_prefix} ${CMAKE_CURRENT_SOURCE_DIR}/${capnp_file} ${TCS_IMPORT_PATHS} ${MP_INCLUDE_DIR}
+      COMMAND ${MPGEN_BINARY} ${CMAKE_CURRENT_SOURCE_DIR} ${include_prefix} ${CMAKE_CURRENT_SOURCE_DIR}/${capnp_file} ${TCS_IMPORT_PATHS} ${MP_INCLUDE_DIR}
       DEPENDS ${capnp_file}
       VERBATIM
     )