If a package depends on other packages these will be built
automatically in the correct order. To see the dependencies of any
package use the dep-list target.
$ make -C gnu/gnupg dep-list
make: Entering directory `/home/gnu/gsrc/gnu/gnupg'
libgpg-error libgcrypt libassuan libksba pth zlib readline
make: Leaving directory `/home/gnu/gsrc/gnu/gnupg'
The dependencies are searched for in the gnu/ subdirectory by
default, with some additional external packages such as zlib in
the deps/ subdirectory.
Note that the dependencies can be more than one level deep,
$ make -C gnu/readline dep-list
make: Entering directory `/home/gnu/gsrc/gnu/readline'
ncurses
make: Leaving directory `/home/gnu/gsrc/gnu/readline'
So, to install a complex package like gnupg use the same
commands as for a simple package,
$ make -C gnu/gnupg
$ make -C gnu/gnupg install