fix(cmake): Ensure proto headers are generated early

When building using the Ninja generator, the headers were not always
generated before first use in dependent projects. This forces them to
be generated by making the xpp target explicitly depend on them.

CMakeLists.txt

@@ -140,11 +140,25 @@ foreach(PROTO_RAW ${PROTO_LIST_RAW})
   endif()
 endforeach(PROTO_RAW)
 
+#
+# Add commands
+#
+set(PROTO_HEADER_FILES "")
+foreach(PROTO ${PROTO_LIST})
+  string(REGEX REPLACE "proto\$" "" PROTO_OUTPUT ${PROTO})
+  set(OUTPUT_FILE ${PROJECT_SOURCE_DIR}/include/xpp/proto/${PROTO_OUTPUT}.hpp)
+  add_custom_command(
+    OUTPUT ${OUTPUT_FILE}
+    COMMAND ${PYTHON_EXECUTABLE} ${PROJECT_SOURCE_DIR}/generators/cpp_client.py -p "${PYTHON_XCBGEN}"
+    ${XCBPROTO_PREFIX}/share/xcb/${PROTO}.xml > ${OUTPUT_FILE})
+  list(APPEND PROTO_HEADER_FILES ${OUTPUT_FILE})
+endforeach(PROTO)
+
 #
 # Create project lib and commands
 #
 file(GLOB_RECURSE HEADER_FILES ${PROJECT_SOURCE_DIR}/include/*.hpp)
-add_library(${PROJECT_NAME} ${HEADER_FILES})
+add_library(${PROJECT_NAME} ${HEADER_FILES} ${PROTO_HEADER_FILES})
 
 target_include_directories(${PROJECT_NAME} PUBLIC ${XPP_INCLUDE_DIRS})
 target_include_directories(${PROJECT_NAME} PRIVATE ${PROJECT_SOURCE_DIR}/include/xpp)
@@ -157,16 +171,6 @@ target_compile_options(${PROJECT_NAME} PUBLIC ${X11_XCB_DEFINITIONS} ${XCB_DEFIN
 
 set_target_properties(${PROJECT_NAME} PROPERTIES LINKER_LANGUAGE CXX)
 
-#
-# Add commands
-#
-foreach(PROTO ${PROTO_LIST})
-  string(REGEX REPLACE "proto\$" "" PROTO_OUTPUT ${PROTO})
-  add_custom_command(TARGET ${PROJECT_NAME} PRE_BUILD
-    COMMAND ${PYTHON_EXECUTABLE} ${PROJECT_SOURCE_DIR}/generators/cpp_client.py -p "${PYTHON_XCBGEN}"
-    ${XCBPROTO_PREFIX}/share/xcb/${PROTO}.xml > ${PROJECT_SOURCE_DIR}/include/xpp/proto/${PROTO_OUTPUT}.hpp)
-endforeach(PROTO)
-
 #
 # Export lists to the parent scope if there are any
 #