how to prevent “directory already exists error” in a makefile when using mkdir

Looking at the official make documentation, here is a good way to do it:

OBJDIR := objdir
OBJS := $(addprefix $(OBJDIR)/,foo.o bar.o baz.o)

$(OBJDIR)/%.o : %.c
    $(COMPILE.c) $(OUTPUT_OPTION) $<

all: $(OBJS)

$(OBJS): | $(OBJDIR)

$(OBJDIR):
    mkdir -p $(OBJDIR)

You should see here the usage of the | pipe operator, defining an order only prerequisite.
Meaning that the $(OBJDIR) target should be existent (instead of more recent) in order to build the current target.

Note that I used mkdir -p. The -p flag was added compared to the example of the docs.
See other answers for another alternative.