So what opens this additional buffer is your command exe ":botright copen", :h :copen is used to open the quickfix window. As you use it no matter how the compilation went, when the compilation is successful the quickfix doesn't contain anything and the buffer looks open.

So the first thing to do is to remove the useless usages of :exe: In exe ":cclose", exe ":update" and exe ":botright copen" you don't need an execute command since the strings to pass to execute are already complete normal commands. So replace these lines by cclose, update and botright open.

Now to prevent the buffer from opening on failed compilation you probably want to move botright copen to a specific condition. It could be like this:

if empty(v:statusmsg)
    let &statusline = "'".obj."':Compiled successfully"
else
    botright copen    
endif

Also a few different ideas:

• As B.Layer pointed out in the comments you can reduce the cyclomatic complexity of your function by refactoring your if conditions
• You don't need to use : in front of normal commands in a vimscript
• I think there is no benefit using the short name of commands in a script
• And I think you don't need to surround your save of makeprg between quotes you should be able to do let save = &makeprg and then execute 'set makeprg = ' . save

So here is how I would refactor your code:

function! makes#Fcompile()
    cclose
    update

    let sou = expand("%:p")
    let obj = expand("%:p:r").s:ObjExt

    " Don't process any further if the compilation is up to date
    if filereadable(obj) && (getftime(obj)>getftime(sou)))
        let &statusline = "'" . obj . "':is up to date"
        return
    endif

    let makeprg_saved = '"' . &makeprg . '"'
    execute "setlocal makeprg=" . s:Compiler
    let v:statusmsg = ''
    execute "make " . s:FCFlags . " " . sou . " -o " . obj

    " Don'r process any further if the compilation was sucessful
    if empty(v:statusmsg)
        let &statusline = "'" . obj . "':Compiled successfully"
        return
    endif

    if v:shell_error !=0
        let &statusline = v:shell_error
    endif
    botright copen
endfunction

So what opens this additional buffer is your command exe ":botright copen", :h :copen is used to open the quickfix window. As you use it no matter how the compilation went, when the compilation is successful the quickfix doesn't contain anything and the buffer looks open.

So the first thing to do is to remove the useless usages of :exe: In exe ":cclose", exe ":update" and exe ":botright copen" you don't need an execute command since the strings to pass to execute are already complete normal commands. So replace these lines by cclose, update and botright open.

Now to prevent the buffer from opening on failed compilation you probably want to move botright copen to a specific condition. It could be like this:

if empty(v:statusmsg)
    let &statusline = "'".obj."':Compiled successfully"
else
    botright copen    
endif

Also a few different ideas:

• As B.Layer pointed out in the comments you can reduce the cyclomatic complexity of your function by refactoring your if conditions
• You don't need to use : in front of normal commands in a vimscript
• I think there is no benefit using the short name of commands in a script
• And I think you don't need to surround your save of makeprg between quotes you should be able to do let save = &makeprg and then execute 'set makeprg = ' . save

So here is how I would refactor your code:

function! makes#Fcompile()
    cclose
    update

    let sou = expand("%:p")
    let obj = expand("%:p:r").s:ObjExt

    " Don't process any further if the compilation is up to date
    if filereadable(obj) && (getftime(obj)>getftime(sou)))
        let &statusline = "'" . obj . "':is up to date"
        return
    endif

    let makeprg_saved = &makeprg
    execute "setlocal makeprg=" . s:Compiler
    let v:statusmsg = ''
    execute "make " . s:FCFlags . " " . sou . " -o " . obj
    execute 'set makeprg=' . makeprg_saved

    " Don'r process any further if the compilation was sucessful
    if empty(v:statusmsg)
        let &statusline = "'" . obj . "':Compiled successfully"
        return
    endif

    if v:shell_error !=0
        let &statusline = v:shell_error
    endif
    botright copen
endfunction