Shell Programming and Scripting

This is driving me crazy. I'm trying to automate some things in my makefile and I'm running into a substitution issue.

I'm trying to automatically create object directories if they don't already exist based on the listed source files found in the $(SRCARM) variable.

SRCARM = dir1/file1.c \
dir2/file2.c \
dir3/file3.c \
main.c

COBJARM = $(SRCARM:%.c=$(OBJDIR)/%.o)

which results in:
COBJARM = output/dir1/file1.o \
output/dir2/file2.o \
output/dir3/file3.o \
output/main.o

These two work, giving me the source and object lists. Now to get all objects' directories so I can create them if they don't already exist.
I'd like to create the following:
COBJARMDIR = output/dir1 \
output/dir2 \
output/dir3 \
output

So I try this:
COBJARMDIRS = $(COBJARM:%/=%)

Which doesn't work. I don't know if I was expecting it to match the first or last /. I just expected it to do something. I tried escaping the slash as well. All I get is the same list of object files. It appears you have to match the entire end of the string to get the substitution to work.

For example:
COBJARMDIRS = $(COBJARM:%ain.o=%)
results in:
COBJARMDIRS = output/dir1/file1.o \
output/dir2/file2.o \
output/dir3/file3.o \
output/m.x

but this:
COBJARMDIRS = $(COBJARM:%main=%)
doesn't replace anything:
COBJARMDIRS = output/dir1/file1.o \
output/dir2/file2.o \
output/dir3/file3.o \
output/main.o

Sorry for the long winded post. I've just been working on this for 2 hours and I'm getting frustrated.

Does anyone know how to do this? Is this even possible?