In following answers I try to guess what are your issues.
I suppose you are setting up a 64 bit environment.
You have to verify that you installed 64 bit gtkmm2 and GTKMM3 packages :
pacman -Q mingw-w64-x86_64-gtkmm mingw-w64-x86_64-gtkmm3.
if not installed, reinstall.
Verify also your installation of lcms2 and FFTWF
Generally make sure that all installation steps for your 64 bit setup are complete.
This time again, you do not provide a lot of information. You have to use a mingw64 shell window to build.