Error compiling GIMP GIT

Hi.
As far as I know, I never had to install any -dev packages (I understand what you mean) in order to compile something. I just had to install the packages and everything was at its place.
If you

pacman -Ql

a package of a library installed on your machine (-Ql gives you the list of files belonging to the package), you can see that .h files are included.

Hope to help

Ciao