build: Remove depend.sh.

build: Remove depend.sh.

This script was rather clumsy, and it referred to gcc rather than
$(CC) as it should be. This commit adds a command to Makefile.real
that generates the .d files directly, which is not more complicated.

Use the opportunity to add -MP to the $(CC) command which generates
the dependency files. This option works around make errors in case
a header file is removed from the tree.

This change gives another speedup of ~15% for make dep.