build: conf: fix target definition for generated headers (#1242)

Build system for static files is broken due to wrong custom command
definition.

This patch uses a custom target instead and fix dependencies for
'flb-static-conf' target.

note: this gist helped to solve the problem:

https://gist.github.com/baiwfg2/39881ba703e9c74e95366ed422641609

Signed-off-by: Eduardo Silva <eduardo@treasure-data.com>

Author: edsiper

gen_static_conf/CMakeLists.txt

@@ -8,24 +8,26 @@ set(CONF_DIR ${PROJECT_SOURCE_DIR}/include/fluent-bit/conf/)
 
 # Iterate list creating custom commands for each entry
 set(OUT_FILES "")
+set(STATIC_DEPS "")
 set(FLB_STATIC_CONF_ARR "")
 set(FLB_STATIC_CONF_ARR_SIZE 0)
 foreach(file ${files})
   # Convert configuration file using xxd-c
   message(STATUS "Processing configuration file: '${file}'")
   get_filename_component(in_file ${file} NAME)
 
-  add_custom_command(
+  add_custom_target(
+    flb-static-file-${in_file}.h ALL
     COMMAND xxd-c
     ARGS
     -i ${file}
     -o ${CONF_DIR}${in_file}.h
     -s ${in_file}
-    DEPENDS ${file}
-    OUTPUT ${in_file}.h
+    DEPENDS ${file} xxd-c
     COMMENT "Generating static configuration file: ${in_file}.h"
     )
   LIST(APPEND OUT_FILES "${in_file}.h")
+  LIST(APPEND STATIC_DEPS "flb-static-file-${in_file}.h")
 
   string(REPLACE "." "_" stname ${in_file})
   string(REPLACE "-" "_" stname ${stname})
@@ -48,4 +50,4 @@ configure_file(
 
 # Register the custom target, this will be a later dependency
 # of fluent-bit-static target
-add_custom_target(flb-static-conf DEPENDS ${OUT_FILES})
+add_custom_target(flb-static-conf DEPENDS ${STATIC_DEPS})