FAST_DEPEND: Similar to r290629, do always depend on headers if 'make depend'
has not ran yet.

This fixes building objects directly, or skipping 'make depend', not generating
required headers first. This case did work without FAST_DEPEND so there's no
reason it should not work here as well.

An example of this can be seen building in gnu/usr.bin/binutils/libbfd
without running 'make depend' first to generate config.h.

Sponsored by: EMC / Isilon Storage Division
MFC after: 3 weeks
X-MFC-With: r290433