Could you please give me the exact error please? Also which operating system are you on?
On my machine, it works fine.

By the way, you don't have to comment on other unrelated bugs to get my attention. If you need me to look on a specific bug, add me to CC list and I'll receive an email.

Sorry, so much to learn, didn't realize i could :) 

I'm on Ubuntu 15.10:
automake 1.15
autoconf 2.69
gcc/g++ 5.2.1
bash 4.3.42

Doing 'make install' i get :

================================================================
ln -s -f ../../icons/Symbolic/12/gimp-default-colors.png 12/gimp-default-colors.png
 /bin/mkdir -p '/opt/Apps/gimp-git/share/gimp/2.0/icons/Symbolic-Inverted/hicolor/12x12/apps'
 /usr/bin/install -c -m 644 12/gimp-close.png ./12/gimp-default-colors.png 12/gimp-linked.png 12/gimp-menu-left.png 12/gimp-menu-right.png 12/gimp-quick-mask-off.png 12/gimp-quick-mask-on.png 12/gimp-swap-colors.png 12/gimp-visible.png 12/gimp-zoom-follow-window.png '/opt/Apps/gimp-git/share/gimp/2.0/icons/Symbolic-Inverted/hicolor/12x12/apps'
/usr/bin/install: cannot stat ‘./12/gimp-default-colors.png’: No such file or directory
Makefile:1124: recipe for target 'install-icons12DATA' failed
make[3]: *** [install-icons12DATA] Error 1
make[3]: Leaving directory '/media/trekstor/gimp-commits/gimp-out/icons/Symbolic-Inverted'
Makefile:1446: recipe for target 'install-am' failed
make[2]: *** [install-am] Error 2
make[2]: Leaving directory '/media/trekstor/gimp-commits/gimp-out/icons/Symbolic-Inverted'
Makefile:605: recipe for target 'install-recursive' failed
make[1]: *** [install-recursive] Error 1
make[1]: Leaving directory '/media/trekstor/gimp-commits/gimp-out/icons'
Makefile:780: recipe for target 'install-recursive' failed
make: *** [install-recursive] Error 1
================================================================

ls -al reports the symlink as 

lrwxrwxrwx 1 foo foo   47 Jan 11 13:34 gimp-default-colors.png -> ../../icons/Symbolic/12/gimp-default-colors.png

but when i change it to have this

lrwxrwxrwx 1 foo foo   47 Jan 11 13:34 gimp-default-colors.png -> ../../Symbolic/12/gimp-default-colors.png

it then works fine and will install.

Could you try a `make clean` and build again?

I just fixed this without having noticed the bug:

commit 3428a5c579102fc0c8e554ce237405e36bf7f946
Author: Michael Natterer <mitch@gimp.org>
Date:   Mon Jan 11 20:34:16 2016 +0100

    icons: use cp not LN_S to "create" the inverted 12/gimp-default-colors.png
    
    Linking it apparently messed up something (identical timestamps?) so
    sudo make install would fail.

 icons/Symbolic-Inverted/Makefile.am | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

just did a new pull rebuilding with your changes.

works