If you want to execute it after every compile then you must change the compile command(s) to do both.

${COMMAND} ${FLAGS} ${OUTPUT_FLAG} ${OUTPUT_PREFIX}${OUTPUT} ${INPUTS}; fromelf ${OBJECT}.o > ${OBJECT}.txt:

The post build step is for doing things that you might want to do after every build. Like converting the linked executable to another format.

If you want the assembly of each file, the easiest option may be to add "-save-temps" as an additional compiler flag. That will cause GCC to leave the .s file rather than delete it when done. It will also IIRC save the .i (pre-processed) source file too.

Also, if you edit your compile command to run too commands, make sure you join them together with && instead of ;. Otherwise if the first command fails and the second succeeds you won't get a build error, but you will have a broken build.

1. run compiler on .c file that produces object (.o) file
   
2. run "fromelf" command on this .o file to produce assembly file
   

I'm not clear if you are trying to get the corresponding assembly for each object file or for the build executable.

If it's for each object and you want to use the post-build step, you will need to get the objects used for the link.
The problem is the post-build step probably won't have dependencies .
You'll need to find another way to get the list.

If it's just for each object , you can get gcc to output the assembly for each source file.
This link explains how: http://stackoverflow.com/a/137074/3312726
So the compile command would be
<asm output command>; <compile command>

If you are using Windows and the makefile recipes are executed by cmd.exe I think you will need to use '&' to separate the commands.

I believed that "Post-build" option serves for such kind of scenarios. Or I've been mistaken ?

10:41:07 **** Build of configuration Debug for project HelloForum ****
make all 
Building file: ../src/HelloForum.c
Invoking: GCC C Compiler
gcc -O0 -g3 -Wall -c -fmessage-length=0 -MMD -MP -MF"src/HelloForum.d" -MT"src/HelloForum.o" -o "src/HelloForum.o" "../src/HelloForum.c"
Finished building: ../src/HelloForum.c
 
Building target: HelloForum
Invoking: GCC C Linker
gcc  -o "HelloForum"  ./src/HelloForum.o   
Finished building target: HelloForum
 
make --no-print-directory post-build
I am the post build description
echo "I am in post build"
I am in post build
 

10:41:07 Build Finished (took 113ms)