Make the _tmp directory before building.

This is more consistent and simplifies the dev build instructions.

Makefile

@@ -64,9 +64,11 @@
# easier to read.
.SUFFIXES:

# Do this before every build.  There should be a nicer way of handling
# directories but I don't know it.
$(shell mkdir -p _bin _release _build/hello _build/oil)
# Make all directories before every build.  There might be a nicer way of
# handling directories but I don't know it.
# NOTE: _devbuild is made by build/dev.sh.  That directory is NOT cleaned with
# 'make clean'.
$(shell mkdir -p _bin _release _tmp _build/hello _build/oil)

ACTIONS_SH := build/actions.sh
COMPILE_SH := build/compile.sh